FIG. 3.

Suppose, now, we have a glass plate with a design of a tulip, with its blossom ruled with 2,000 lines to the inch, its leaves ruled with 2,400 and the pot in which it is growing ruled with 2,750 lines, and place this plate before the lens. On looking through the hole we shall see a red tulip with green leaves growing in a blue pot. Thus we see how it is possible to produce a colored picture by means of diffraction lines, which are in themselves colorless. Those portions of the plate where there are no lines send no light to the eye and appear black.

We have, now, to consider how this principle can be applied to photography. That photographs which show color on this principle can be made depends on the fact that a diffraction grating can be copied by contact printing in sunlight on glass coated with a thin film of bichromated gelatine. The general method which I have found best is as follows. Three gratings ruled on glass with the requisite spacing were first prepared.* To produce a picture in color, three negatives were taken through red, green and blue color filters in the usual manner. From these three ordinary lantern-slide positives were made. A sheet of thin plate glass was coated with chrom gelatine, dried, and cut up into pieces of

*These gratings were ruled for us on the dividing engine at Cornell University, through the courtesy of Professor E. L. Nichols.

the sensitive film in contact with the ruled surface of the 2,000-line grating, and the whole covered with the positive representing the action of the red light in the picture. An exposure of thirty seconds to sunlight. impressed the lines of the grating on the film in those places which lay under the transparent parts of the positive. The second grating and the positive representing the green were now substituted for the others and a second exposure was made. The yellows in the picture being transparent in both positives, both sets of lines were printed superposed in these parts of the picture, while the green parts received the impression of 2,400 lines to the inch only.

The same was done for the blue, and the plate then washed for a few seconds in warm water. On drying it appeared as a colored photograph when placed in front of the lens and viewed through the hole in the screen. Proper registration during the triple printing is secured by making reference marks on the plates. A picture of this sort once produced can be reproduced indefinitely by making contact prints, since the arrangement of the lines will be the same in all of the copies as in the original. The finished picture is perfectly transparent and is merely a diffraction grating on gelatine with variable spacing. In some parts of the picture there will be a double grating, and in other parts (the whites) there will be a triple set of lines. Having had some difficulty in getting three sets of lines on a single film in such a way as to produce a good white, I have adopted the method of making the red and green gratings on one plate and the blue on another, and then mounting the two with the films in contact. It is very little trouble to multiply the pictures once the original redgreen grating picture is made.

The pictures are viewed with a very simple piece of apparatus shown in Fig. 4,

consisting of a lens cut square like a reading glass, mounted on a light frame provided with a black screen perforated with an eye hole through which the pictures are viewed.

FIG. 4.

The colors are extremely brilliant, and there is a peculiar fascination in the pictures, since, if the viewing apparatus be slowly turned so that its direction with reference to the light varies, the colors change in a most delightful manner, giving us, for example, green roses with red leaves, or blue roses with purple leaves, a feature which should appeal to the impressionists. The reason of this kaleidoscopic effect is evident, for, by turning the viewing apparatus, we bring the eye into different parts of the overlapping spectra.

It is possible to project the pictures by employing a very intense light and placing a projecting lens in place of the eye behind the perforation in the screen. Of course, a very large per cent. of the light is lost; consequently great amplification cannot well be obtained. I have found that sunlight gives the best results, and have thrown up a three-inch picture on a four-foot sheet, so that it could be seen by a fair-sized audi

ence.

By employing a lens of suitable focus it it possible to make the viewing apparatus binocular, for similar sets of superposed spectra are formed on each side of the central image by the gratings, so that we may have two eyeholes if the distance between the spectra corresponds to the interocular distance.

It is interesting to consider that it is theoretically possible to produce one of these diffraction pictures directly in the camera on a single plate. If a photographic plate of fine grain were to be exposed in succession in the camera under red, green and blue screens, on the surfaces of which diffraction gratings had been ruled or photographed, the plate on development should appear as a colored positive when seen in viewing apparatus. I have done this for a single color, but the commercial plates are too coarse-grained to take the impression of more than a single set of lines. With specially-made plates I hope to obtain better results.

UNIVERSITY OF WISCONSIN.

R. W. WOOD.

THE MENTAL FATIGUE DUE TO SCHOOL WORK.

THE meaning of the results obtained by the different investigators of fatigue among school children has been much confused because either the experimenter has not proved that what he measured was fatigue at all or has so arranged his experiments that the influence of practice on the one hand, and of unwillingness and lack of interest on the other, have not been discounted. Especially when, as has so often been the case, the teacher gives the work as a part of the school routine one may be measuring only a conventional habit of the school children of doing less at a certain time of day, or an unwillingness to work due to ennui. What a person does do need not be a measure of what he could do.

The experiments a summary of which follows were devised in order to get an answer to the question: "Does the work of a school-session fatigue the pupils mentally, make them really less able to do mental work than they were at its commencement, and, if so, to what extent?"

The method was to give to a sufficient

number of scholars a certain test which would measure their ability (in a certain direction, at least) to do mental work, early in the school session, and then to give this same test to a different lot of children of approximately equal general maturity and ability late in the session. The influence of practice is thus entirely obviated, as the scholars do not have the same sort of work twice. In order to save the results from being vitiated by differences in the general ability of the students, four differerent tests were used, and the pupils who had two of these tests early had the other two late, while those who had the first two late had the other two early. The influence of possible differences in the average ability of the two sets of students can thus be estimated. In order to make sure that the willingness and interest of the pupils was a constant except in so far as due to causes outside our control, all the tests were given by myself.

The work given was: (1) a set of multiplication examples to be done in a given time; (2) a page of printed matter full of mis-spelled words which were to be marked in a given time; (3) two sets of nonsense syllables to be written from memory after a ten seconds' look at them, and (4) two sets of figures and one set of simple forms (e. g., square, triangle) to be written from memory in the same way.

About 150 children (four classes) were given 1 and 3 early and 2 and 4 late. An equal number of children in the same school grade were given 2 and 4 early in the school day, the other half late. In order to eliminate the influence on the work which excitement at my first visit, or being used to me or being tired of me at my second visit, might cause, I made my first visit coincide with an early test in half the classes and with a late test in the other half. The early tests were all given between 10 minutes and 40 minutes after the opening of school in

the morning, while the late tests were given between 40 minutes and 10 minutes before the close of school, half of them at the close of the morning and half at the close of the afternoon session.

Thus any general decrease in the amount or accuracy of the late work will be due to mental fatigue, or to some aversion to work caused by the school day and quite apart from the aversion to conventional routine work or to some factors yet to be demonstrated. And if there is no difference we can say with assurance that the day's work has not decreased the child's ability to work, that though he may in school do less in the latter part of the day it is not in any wise due to real exhaustion, to a lowering of his mental energy.

As a fact what difference there was between the early and late work was in favor of the latter. The multiplication test was given to 152 scholars early and 144 late. After reducing the amount done and mistakes made by the 152 to what would have been done by 144, and comparing the results obtained with the work of the 144 who had the test late, we find that the latter did nearly 14% more work and made less than 5% more mistakes.

The spelling work was given to 152 pupils early and 146 late. After estimating the work of 146 early pupils on the basis of what the 152 did and comparing with the 146 late, we find that the latter worked through 99% as many lines, marked about 2% more words and marked 2% more words which should have been left unmarked.

The nonsense syllables were used with 152 pupils early and 148 pupils late. When reduced to an equality in numbers the late pupils did 9715% % as well as the early.

The figures to be remembered were given to 152 pupils early and 145 late. When reduced to an equality in numbers the late pupils did 89% as well as the early.

0

The spelling test was given to 135 early and 128 late. When evaluated for 127 children the results show the latter to have worked through 92,7 % as many lines, to have marked of 1% more words, and to have marked wrongly 87 % as many words. Taking together the work of all the children tested, we find that those who did the work late worked through 94% % as many lines, marked 15% % more words and marked wrongly 93% % as many.

The figure test was given to 156 children early and to 152 late. After reducing the results of the 156 to a basis of 152 we find that those who had the tests late did 17% better.

Taking together the work of all the children tested, we find that those who had the test late did almost 2% better than those who had it early.

Besides these three tests, which are of the same sort as some of those given to the first lot of children, there was given to this second lot a test with letters similar to the

figure test. This test was given to 140 children early and to an equal number late. Those doing the work late did 97 % as well as those who had it early.

The factors mentioned as influencing the work of the first set of children were largely counterbalanced by factors at work in the second; one, however, should be mentioned. A certain circumstance probably lessened the work of one class (of 30) of the first lot of children during an early spelling test. So the early work in this test should probably be reckoned about 2% higher. On the whole these additional data render more probable our previous conclusion that "the mental work of the school day does not produce any decrease in the ability to do mental work." A glance at the following table, which summarizes the more important data, shows this better perhaps than the detailed accounts already given.

cle will require four entries, on an average one according to the author, the others according to the subjects treated of in the paper.

It is proposed also to print, at the head of each of the cards or slips, distinctive symbols to indicate the science and particular sub-division of the science to which the paper refers.

There can be no doubt but that, to satisfy the needs of workers in laboratories, the plan of having a card catalogue of subjects is by far the most satisfactory. A book catalogue would be practically useless except to a student looking up references for historical reasons, and should, therefore, be kept in a general library, and not in a laboratory library. For the use of workers in laboratories the subject card catalogue would be of the greatest importance, as everyone knows who has ever kept one. It is of great use to the director of the laboratory in the saving of time and brain matter, because he no longer needs to remember all articles which have appeared, and to the student or investigator in keeping him informed of all that is going on in his particular line of work. From the standpoint, then, of Physics there can be no doubt but that it would be desirable for the International Committee to print all three catalogues, the book catalogue and the two card catalogues; and of these the card catalogues should be kept, it seems best, in the laboratory itself, or at least in such a situation as to be ready for use by all the students.

No suggestions are asked by the Committee concerning the division of the sciences or the classification proposed; and, in fact, this matter is of secondary importance. The plan is to have the assistants and the clerks in the Central Office in London make a division of the titles according to subjects and to label the cards and slips in some definite way; so that anyone, although ignorant of the subject-matter, can arrange

the cards easily and quickly when they are received.

Each card in Physics is to be marked with the letter 'D,' and each subject card is to have, further, a number, such as '5410,' which signifies the particular subdivision to which the subject has reference. In this particular case the 5 would indicate the primary division, 'Light;' the 4 the subdivision, 'Polarization;' the 10 the special subject, 'Methods of Producing Polarized Radiation.'

According to this system Physics is divided into seven primary divisions,' socalled, namely: Bibliography and Dynamics; Heat; Mechanical and Thermal Effects of Contact and Mixture; Vibrations, Waves and Sound; Theories of the Constitution of the Ether and of Matter; Light, including Invisible Radiation; Electro-magnetism.

'Bibliography and Dynamics' is subdivided into seven sections: Bibliography of Physics; Dynamics in General; Dynamics of a Particle and Rigid Dynamics; Elasticity; Hardness, Friction and Viscosity; Dynamics of Fluids; Measurements of Dynamical Quantities.

'Heat' is divided into seven sections: Temperature and Thermometry; Calorimetry; Determination of the Mechanical Equivalent of Heat; Fundamental Laws of Thermodynamics; Thermal Conduction and Convection; Changes of Volume and of State (Experiment and Theory); Radiation.

'Mechanical and Thermal Effects of Contact or Mixture' is divided into five sections: Friction; Capillarity; Diffusion; Transpiration and Mechanical Permeability; Imbibition and Surface Condensation of Gases; Solution and Osmose.

'Vibrations, Waves and Sound' is divided into five sections: Theory and Observation of Harmonic Vibrations; Theory of Wave Motion; Sound; The Sensation of Sound; The Physical Basis of Music.