516 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Theory of the Camera Lucida.* — The first ten pages of Dr. E. 

 Giltay's paper deal with the th e-^ y of lenses, nodal points, &c., the 

 constitution of the eye (with a diagram of the cornea, lens, and retina), 

 and contain a discussion of how the image is formed in the eye, 

 whilst the last six pages are devoted to a consideration of the use of 

 lenses between the pencil and the eye (previously published by the 

 author, and noted in this Journal, III. 1883, p. 278). In the rest 

 of the paper the author discusses the best conditions for illuminating 

 the field of view and the drawing paper. 



Take first the case of a white chalk pencil on a black slate. Let 

 fig. 97 represent the image on the retina of the field of view with 

 illumination w and the object with illumination 8, so that w is great 

 in comparison with 8 ; let fig. 98 represent the image of the slate with 



Fig. 97. 



Fig. 98. 



Fig. 99. 



illumination 8' and the chalk pencil with illumination w'. When the 

 two are superposed fig. 99 is the result. The pencil with illumina- 

 tion 8 -j- w' will always be clearly seen upon a faintly illuminated 

 object of which the brightness is 8 -f- ^' 5 whether it is also clearly 

 visible upon the background will depend (since 8' is small) upon the 

 relation between to and w' ; it will be if w is small in comparison with 

 to'. If CO is too great in comparison with <o' it must be diminished. 



Fig. 100. 



Fig. 101. 



The case of a dark pencil upon white paper is represented in 

 figs. 100 and 101, where <o' is now the illumination of the paper and 8' 

 of the pencil. As before, whether the pencil will be easily visible 



Zeitschr. f. Wiss. Mikr., i. (1884) pp. 1-23 (10 figs.). 



