Photographic Delineation of Microscopic Objects. 159 



The body of the microscope being brought to the horizontal 

 position must be inserted into the front of an ordinary portrait 

 camera, from which the lens has been previously removed. In 

 the absence of a special adapter, the aperture round the tube 

 must be stuffed with some convenient material so as to exclude 

 light. The image of the object is then to be accurately focussed 

 on the ground-glass by means of the ordinary coarse and fine 

 adjustment- screws of the microscope.* After focussing, how- 

 ever, it will be found necessary to make a trifling alteration in 

 the adjustment, for the object-glasses being made with an 

 " over-correction/' in order to compensate for the " under- 

 correction " of the eye-piece, their visual and chemical foci do 

 not correspond; and thus the actinic rays are brought to a 

 focus slightly beyond the visual rays. On this account the 

 object-glass will need a certain amount of depression varying 

 with the power, and the higher the power the less alteration 

 will be required ; usually with a quarter of an inch objective 

 the chemical and optical foci are so nearly coincident that the 

 difference may be overlooked in practice. The amount of de- 

 pression required for each lens can only be ascertained by 

 repeated experiment, but the following data which apply to my 

 own object-glasses (Powell and Lealand's) may be taken as an 

 approximation. The one inch glass requires a depression 

 amounting to one turn and a half of the fine adjustment screw 

 (about one seventy-fifth of an inch.) The half inch requires 

 about half a turn of the same screw. 



The most satisfactory illumination is a strong sunlight 

 reflected directly upon the object by the concave mirror. Light 

 reflected from a white cloud opposite the sun, will indeed 

 answer the purpose, but the time of exposure is necessarily 

 greatly increased, and the impression when obtained is much 

 inferior in point of brilliancy and distinctness. 



The " collodion " process is doubtless the best that can be 

 used for microscopic purposes. Indeed, if the direct sunbeam 

 be employed as the illuminating agent, no good result can be 

 obtained with a less sensitive material, for the situation of the 

 image on the prepared plate is continually altering with the 

 altering position of the sun. The time of exposure must differ 

 considerably according to the intensity of the illumination, the 

 medium in which the object is mounted, and the nature of the 

 object itself. When using the direct rays of the sun I have 

 generally found from fifteen to forty-five seconds sufficient for a 

 collodion negative. 



Recent discoveries, by means of which sensitive plates may 



* It is not the aim of this paper to explain the details of ordinary photo- 

 graphic manipulation. For information on these points, the reader must consult 

 some one of the numerous manuals of photography. 



