862 



SCIENCE. 



[N. S. Vol. IX. No. 234. 



consisting of a lens cut square like a read- 

 ing glass, mounted on a light frame provided 

 with a black screen perforated with an eye 

 hole through which the pictures are viewed. 



The colors are extremely brilliant, and 

 there is a peculiar fascination in the pic- 

 tures, 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 efifect is 

 evident, for, by turning the viewing appa- 

 ratus, 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 ; con- 

 sequently 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 cou^ld 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 cen- 

 tral 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 photo- 

 graphic plate of fine grain were to be ex- 

 posed 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. 



E. W. Wood. 



University of Wisconsin. 



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 be- 

 cause either the experimenter has not 

 proved that what he measured was fatigue 

 at all or has so arranged his expei'iments 

 that the influence of practice on the one 

 hand, and of unwillingness and lack of in- 

 terest on the other, have not been dis- 

 counted. 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 an- 

 swer 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 



