222 THE ROYAL SOCIETY OF CANADA 



and in the second place, of investigating the behaviour of these maxima 

 with changes in the intensity of the radiation used, tiie following obser- 

 vations were made. 



When it was found that all of the chief characteristics might be 

 investigated b}^ the apparatus to be used in the case of number I, and 

 in view of the fact that it appeared to be an original method, it was 

 decided to investigate all three cases. It was thought that the accuracy 

 with which the well known phenomena were established might indicate 

 the reliability of the results obtained in the first case. 



Historical * 



Mandelstamm f appears to have been the first to investigate the 

 color sensitiveness of the human eye for slight differences in the wave 

 length of light observed. By shifting the plates of an ophthalmometer 

 so as to give a just perceptible color difference, he obtained a means of 

 observing sensibility in different parts and obtained in this way maxima 

 of sensibility in the region of the D and F lines. Dobrow^olosky J came 

 to the same conclusion by similar means. Peirce^ investigated the 

 sensitiveness of the eye to slight differences of color by having two 

 identical spectral bands one immediately above the other, the upper 

 one of which might be shifted. The object of the experiment was to 

 see how small a displacement could be infallibly detected and named in 

 direction by the observer in different parts of the spectrum. He found 

 maxima situated similarly to those found by other investigators. More 

 accurate results were first obtained by Konig and Dieterici.^ Uhthoff^ 

 also investigated the differences in wave length for just observable 

 color differences. Brodhun,' himself color blind, gives measurements 

 after the method of Konig for mean intensities. Exner* also gives a 

 single service of measurements on a widely dispersed spectrum. In this 

 connection also Steindler* reports observations upon twelve subjects. 

 A spectrum with a dispersion of aljout 85 cm. at the point observed was 

 obtained by means of an arc light and a concave grating. The light 

 from the spectrum at this point fell upon two totally reflecting prisms 

 placed vertically one above the other and after reflections from a second 



♦Historical references taken from Dr. Stcindler's paper. 



tGrafe's Archiv: Bd. 13, p. 399. 



JEbenda, Bd. IS, p. 99. 



* American Journal of Science, Vol. 20 (1883), p. 299. 



■^ Annalen dcr Physik und Chemie, Bd. 22, p. 579. 



" Grafe's Archives, Bd. 34, 4, p. 1. 



' Zeitschr. fiir Psych, vind Phys. Bd. 34 (1892), p. 89. 



*L. c. p. 875. 



«WienSitz.: lia, 115, pp. 1-24, 19()G. 



