338 MICROSCOPIC PHOTOGRAPHY. [MEMOIR XXV. 



MEMOIR XXV. 



ON MICROSCOPIC PHOTOGRAPHY. 



CONTENTS : Method of making microscopic photographs by condensed 

 sunlight. Specimens of the art. 



WHEN giving courses of lectures on Physiology in 

 the University of New York, I found it very desirable 

 to have photographic representations of various micro- 

 scopic objects. There are many such objects, costing 

 much time and trouble in their preparation, which it is 

 very difficult, if not impossible, to preserve. A photo- 

 graph is their best substitute. 



I used as the sensitive surface daguerreotype plates, 

 for this was previous to the invention of the collodion 

 process. The daguerreotype plate is, however, much less 

 sensitive than the collodion film. My first attempts 

 were to copy the objects by lamplight, but this was 

 found to be not sufficiently intense. Even after very 

 long exposures with low powers the impressions ob- 

 tained are unsatisfactory. 



On employing a beam of sunlight impressions could 

 be quickly obtained, and by concentrating the beam 

 with a condensing lens the object could be sufficiently 

 illuminated to suit any magnifying power. A fine mi- 

 croscope was used to give the image. 



Two difficulties were, however, to be provided for: 

 1st. If the condensing lens be large, the heat at its focal 

 point may be so high as to injure or even destroy the 

 object. 2d. It is not easy to find the chemical focus, for 

 it does not coincide with the visual one. 



