1890.] NEW YORK ACADEMY OF SCIENCES. 77 



work may thus become intelligent, and he is fitted to cope nn- 

 derstandingly with unforeseen obstacles. The mysterious action 

 of light is, in itself, a subject of fascinating interest to the 

 youngest student. It is important to make him a})preciate the 

 universal character of its chemical action, — that this is not con- 

 fined to photographic gelatin plates, but has already become 

 familiar to him in numberless instances ; e.g., in the tanning 

 of his own skin where exposed to the summer^s sun, and its 

 whiteness where protected by clothing, in the variegated coloring 

 and streaking of flower-petals where exposed to or shielded from 

 the sunlight, in the bleaching of dyed curtains, muslins, etc., 

 long exposed to light, and many similar facts. 



The simplicity of the apparatus needed to obtain fair results 

 was then explained and illustrated. 



The more scientific and difficult applications of photomicro- 

 graphy were then discussed. 



The great advantage of the yellow light of the common lamp- 

 flame was explained, in connection with the use of color-sensi- 

 tive plates and the proper color-screens. In the photography 

 of bacteria and similar minute objects under higli-power lenses, 

 reference was made to the great value of the use of the simple 

 little magnesia-beads, first suggested by Prof. Roux, in place of 

 the ordinary lime-cylinders in the oxyhydrogen light. Their 

 construction was explained, with the further recommendation to 

 flatten slightly the end of the platinum wire, before its intro- 

 duction into the magnesia-bead, in order to insure more perfect 

 adhesion. A finer jet should be also substituted, in place of that 

 used with a lime-cylinder, both for economy of gas, more intense 

 illumination, and concentration of the flame on the bead, without 

 danger of melting away the end of the platinum wire. 



Attention was given to some of the peculiar difficulties yet 

 attending the worker who has the most perfect results in^view, 

 viz. : — 



1. The scarcity of perfect mounts of most microscopic objects^ 

 such as could be used to obtain unexceptionable photographs. 



2. The complication of apparatus required for successful 

 photography of living and moving organisms. 



3. The difficulty in preparation of stable gelatin plates, sen- 

 sitive to the red rays of the spectrum. 



4. The interference of vibration and that of the Brownian 

 motion in minute objects mounted in liquid preservatives. 



5. The photography of opaque objects, especially those with 

 brilliant or metallic lustre. A new method for this purpose was 

 explained, with illustrations. 



G. Decomposition of gelatin plates during washing in hot 



