Light Filters for Photomicrography. By E. Moffat. 21 



separate cells, or mixed together, when a muddy compound will be 

 produced, but by adding acetic acid drop by drop this will clear 

 up, and a fine permanent filter will be the result, this being excellent 

 also for visual work. 



My experience has been that with these liquid filters a far 

 superior result is obtained than with gelatin-stained films, as the 

 latter when rubbed stop a considerable amount of light. The 

 liquid filters pass a maximum of light, and so reduce exposure to 

 a minimum, and at the same time act as heat-absorbing troughs, 

 enabling the Microscope to keep longer in focus. Monochromatic 

 light is hardly practicable unless the arc lamp is used, and, after 

 all, a bichromatic light is ample for nearly all purposes, and by 

 working with the two chemicals named much good work can be 

 done. A saturated solution of copper acetate in a fairly deep cell, 

 say 25 mm., will cut out the red end of the spectrum and also the 

 orange beyond the D line. A strong solution of potassium 

 bichromate will absorb the spectrum from the violet end through 

 the blue and beyond the F line. A special case may arise where 

 a red sensitive plate and a red filter are required, such as in a 

 faintly-stained methylen-blue preparation, where the absorption 

 band is principally about the C line ; but in practice a good nega- 

 tive can generally be got of this by a deep orange filter and a 

 yellow sensitive plate — these plates being exceptionally good for 

 photomicrographic work, and generally giving greater contrast than 

 the plate sensitised to the whole spectrum. 



As before stated, insect dissections, and similar objects of a 

 pale-yellow or straw colour — the chitinous substances assuming 

 such tints when mounted in balsam — might be well represented by 

 Bismark brown (Yesuvian), which has an absorption band from the 

 violet end of the spectrum to the F line. A successful result can 

 in most cases be obtained by a filter of gentian-violet, which has an 

 absorption band in D towards the yellow, using an ordinary slow 

 plate and giving a minimum exposure. 



Excessively rapid dry-plates I have found to be of no advantage, 

 as there is a greater danger of chemical and light fog, owing to the 

 time usually required in development of photomicrographic nega- 

 tives in comparison with field or landscape work, much greater 

 contrast being demanded. Personally, I have found that when the 

 first appearance of the image is from 2^-3 minutes, and is com- 

 pleted in about 15-18 minutes, the best negatives are obtained, 

 pyro soda, with a large quantity of potassium bromide, being the 

 developer used. 



The accompanying photographs were taken on Barnet ortho- 

 chromatic plates and printed on glossy bromide paper, the light 

 used being a Nernst lamp, 1 ampere on 100-volt circuit, with the 

 addition of an ordinary lantern condenser, the exposure being 

 marked on each. 



