PHOTOGRAPHY TO THE MICROSCOPE. 263 



on this point. With light from a white cloud I have ob- 

 tained negatives in from one to three minutes^ with 1-incli 

 and i-inch objectives ; though not highly magnified, they 

 were inferior to those taken with sunlight. Shadbolt ob- 

 tained negatives by concentrating the light of a small cam- 

 pheue lamp on the object with two lenses. Wenliam, in re- 

 peating this experiment, met with no success. I concentrated 

 the light of two flames of a " burning fluid '' lamp, with a 

 bull's-eye condenser, on the object, employing the 1-inch 

 objective without an eye-piece, and obtained, with several 

 samples of collodion from different manufactures, absolutely 

 no image at all, after an exposure of five minutes. With 

 samples prepared by myself tolerably intense negatives were 

 obtained in four minutes. With the -J--inch objective a faint 

 image Avas obtained in the same time. For my own work 

 direct sunlight is always employed. 



It is well known that the proper display of microscopic 

 objects requires that great attention should be given to their 

 illumination, not only in degree, but in kind ; much has been 

 written on this subject, and an astonishing amount of labour 

 bestowed on it. All this applies with double force to the 

 illumination preparatory to the introduction of the sensitive 

 plate ; refined methods here find a most useful application. 

 If the power employed be under 100 diameters, the plane or 

 concave mirror will answer, if the stage be provided with a 

 diaphragm-plate having apertures of different size. The mirror 

 shovild be most carefully adjusted, so that the maximum of 

 distinctness in the image on the ground glass is obtained. 

 I was kindly furnished by Professor Charles A. Joy with a 

 silvered mirror which Liebig presented to him some months 

 ago while on a visit in Europe. It furnished brighter light 

 than the ordinary amalgam mirror j the use of Liebig's 

 mirrors for this purpose, as well as for ordinary microscopic 

 work,, is to be recommended. With powers from 100 to 2000 

 diameters, a condenser of some form is needed ; for powers 

 from 100 to 400 diameters, an achromatic condenser, adjust- 

 able by rack work, was used. Such a condenser must be 

 provided with a series of diaphragms, having circular apertures 

 diff'ering in size ; also with a set of central stops for annular 

 oblique illumination. 



Trial alone will settle which aperture gives the clearest 

 image in any particular case. As the lenses of this condenser 

 Avere not large, I constructed, for powers from 400 to 2000 

 diameters, a Wollaston doublet, with an angular aperture of 

 44°, the lenses being '5 and "6 of an inch in diameter. This 

 condenser, Avhen provided with a similar set of diaphragms. 



