ANATOMICAL TECHNIQUE 463 



serious of these is the discrepancy between the chemical and visual 

 focus. As a result of this shortcoming, a picture sharply focused 

 to the eye appears dull and indistinct upon the photographic plate. 

 In the case of apochromatic objectives and compensation oculars 

 this optical defect is less apparent than in ordinary so-called achro- 

 matic lenses and Huygenian eyepieces. The lack of correspond- 

 ence of visual and chemical foci becomes practically negligible 

 where certain color screens are used in securing the photographic 

 image. In some instances, particularly in the photography of 

 unstained material, lenses of rock crystal are of value, since they 

 shut out less actinic light than do lenses of glass. The great cost, 

 however, of lenses of this construction renders them unavailable 

 for most laboratories, and in any case their value is by no means 

 proportionate to their price. 



Since microscopic sections are generally colored either natu- 

 rally or by means of special stains, photographic plates which are 

 sensitive to color are necessary in photomicrography. There are 

 various types of such plates which are available for different pur- 

 poses. It has been found that certain chemical substances, par- 

 ticularly anilin dyes, possess the valuable property of greatly 

 increasing the sensitiveness of the photographic plate to colors. 

 The photographic negative is ordinarily produced on a gelatin 

 surface mounted on glass. The gelatin covering is known as the 

 emulsion and contains bromide of silver precipitated in its sub- 

 stance. The silver bromide is rendered more or less sensitive to 

 light by boiling in the presence of ammonia, on the one hand, or 

 by adding an excess of bromide of potash, on the other. It is most 

 sensitive to the radiations of the violet and ultra-violet region of 

 the spectrum. Consequently the common photographic plate will 

 not give good results in the case of sections stained with red, green, 

 or even very dark blue dyes. This original defect of the photo- 

 graphic plate has been almost entirely removed in recent years by 

 the use of anilin sensitizers. For example, erythrosin possesses 

 the property of rendering plates soaked in a weak solution sensi- 

 tive to greens. Plates which are so treated are ordinarily known 

 as isochromatic. The appellation is, however, obviously a mis- 

 nomer, since they are only in a very slight degree, if at all, sensitive 



