864 ATLAS 



yellow ; the orange screen (red and yellow) is printed in blue. Having such screens, we find 

 that they are valuable for ordinary photomicrography where the results are to be viewed in 

 monocrome, as we are enabled to emphasize certain points much better than when dependent 

 on the orthochromatic yellow screen alone. Now, the study of stained sections through the 

 microscope will impress the fact upon our minds that the great majority are stained with two 

 colors at most; and as the section before staining was white, or nearly transparent, the need of 

 using three colors for its reproduction does not exist. It will suffice in many cases, therefore, 

 to use but two screens taking a negative with each and leave the reproducer to mix his 

 printing inks so that two impressions will give a satisfactory result. Most of the photomicro- 

 graphs in this Atlas were taken in this manner, only a few in three colors, and the remainder in 

 one or two, as the subject seemed to demand. By eliminating one process in this way through 

 all steps of the work, there is a great gain in every direction, less liability to 'error in the 

 original negatives, sharper impressions in the reproduction, and a saving 'in time and cost 

 throughout. 



An isochromatic plate was used with all colored sections, but not one specially dyed for 

 each particular region of the spectrum that the screens allowed to pass through. Development 

 was with glycin, the plates being allowed to stay undisturbed in the tray until developed, with- 

 out rocking. Two negatives were taken on the same plate, usually the red and blue printing 

 negatives, by means of a sliding plate-holder, and thus, as they were afterward developed 

 together, each exposure had to be carefully timed to the ratio of its respective screen. The 

 screens had the following ratio to each other, green, 3 ; red, 5 ; violet, 9, the electric arc 

 being the illuminant. The exposure varied according to the objective, density of object, magni- 

 fication, position of substage condenser, and length of camera draw, between -^ of a second and 

 five minutes, the usual time being three to ten seconds. As for illustrative purposes, flatness 

 of field and depth of focus were important points, use was made of an objective of low power 

 combined with a high eyepiece and sufficient camera length to obtain the desired magnification. 



EDWARD LEAMING. 

 March, 1905. 



