Photomicrographs and Lantern Slides 139 



slides and contrast plates of larger sizes have the same speed and, 

 consequently, one can determine the length of exposure by using a 

 cheap lantern slide. In making tests, it will save both time and 

 money to expose for 5 seconds and then push in the dark slide so as to 

 cover a part of the plate; then expose 5 seconds longer and push the 

 slide in a little farther, etc. In this way you can make four or five 

 exposures on a lantern slide plate, showing exposures of 5, 10, 15, 20, 

 and 25 seconds, the first exposure being 5 seconds and the last, 25 

 seconds. A print from such a negative is valuable, since it enables 

 one to judge very accurately the printing quality of the various 

 exposures. 



The ordinary filters with a bichromate of potash color, used in 

 out-of-door work, are good, especially when used in addition to some 

 filter suited to the particular stain. We have found a yellowish-green 

 filter very good for most iron-alum haematoxylin stains and also 

 for the safranin, gentian-violet, orange combination. We prefer a 

 stained-glass filter, because it is constant and careful records will soon 

 enable one to guess with considerable precision, while liquid filters 

 vary so much that records have comparatively little value. Filters, 

 of course, lengthen the exposure. The strong photographic filter 

 mentioned in the descriptions of several of the photomicrographs in 

 this book increases the exposure 15 times. 



With fast plates and without filters, a strong light will allow 

 exposures of a fraction of a second, but we have had no success under 

 such conditions. 



The Abbe condenser, which should not be used at all with low 

 powers, is very useful with objectives of 8 mm. focus and all higher 

 powers, especially if the condenser is achromatic. If the con- 

 denser is not achromatic, it is sometimes a good plan to remove it and 

 in its place put a 16 mm. objective, or, for very high powers, even 

 an 8 mm. objective. The condenser may be fastened into the con- 

 denser sleeve by an improvised ring or collar. Zeiss makes a collar 

 for this purpose. 



In addition to the Abbe condenser, there should be another, 

 placed between the microscope and the light. For this purpose, 

 the large condenser from a projection lantern may be used. For 



