206 THE PHOTO-MICROGRAPHY OF 



National Museum in Washington, D. C., for valuable advice and 

 instruction, which his rare comprehension of the difficulties to be 

 overcome enabled him to give me, and for the facilities afforded 

 me in the laboratory under his charge. 



The limits of this paper will not permit of my describing 

 minutely the details which belong to ordinary photography, and 

 so many are already accustomed to the treatment of usual subjects 

 that it would be needless to do so. I shall therefore only endea- 

 vour to indicate the points of special interest and the necessary 

 apparatus. 



For those practically unacquainted with photography, I would 

 suggest that a few lessons from a professional photographer will be 

 of immense value, saving much time and mental perturbation in 

 ascertaining the best manner of working. 



The necessary parts of the apparatus are not numerous, and 

 need be but very simple. They consist of a microscope, a light, 

 a condensing lens, a photographic camera with a plate-holder, 

 plates and a few chemicals, and a room to work in. The micro- 

 scope should have good objectives, for they will be subjected to a 

 very severe trial, and the most judicious treatment fails to get 

 good results with poor objectives. The qualities specially desir- 

 able are achromatism, good definition, penetration, and a flat 

 field. The two latter have been the most difficult to obtain, in 

 my experience. The penetration may be improved by inserting a 

 diaphragm behind the posterior combination of the objective at 

 the point of the greatest convergence of the rays. This point is 

 most easily ascertained by sliding the diaphragm up and down 

 until the proper spot is reached. The use of a diaphragm dimi- 

 nishes the amount of light ; but, with low powers where the great- 

 est penetration is necessary, the amount of light admitted into the 

 objective is so large that this loss is of no consequence. It would, 

 doubtless, be convenient to have apochromatic glass ; but the 

 difference between the chemical and visual foci may be remedied 

 by the use of a blue cell. In working without an eye-piece, the 

 objective should be screwed to the end of a short, wide tube — say, 

 about six inches long and two inches wide, well blackened in its 

 interior. 



As to the relative merits of working with or without an eye- 



