144 PRACTICAL PHOTOMICROGRAPHY. 



an image as that of amphipleura pellucida should consist of 

 pure white and absolute opacity, and the margins of black and 

 white must be sharply cut. The microscopic image is really 

 essentially an image of lines and points, but in many cases we 

 have to show our lines so blended that the appearance is not one 

 of lines but of masses resembling the masses of a portrait or 

 landscape photograph. If we have such masses in a slide, 

 they must be full of differentiation, or " half tone," as it may 

 be called ; anything approaching opacity here will be fatal. 

 As instances of masses we may cite low power images of 

 insects, physiological preparations, homogeneous tissues of 

 any kind, "resolvable," perhaps, but not for our purpose 

 u resolved." All such masses, then, must show half tone. 



Essential, we have now seen, to a perfect slide are : Clear- 

 ness of highest lights ; opacity of resolved lines and points ; 

 half tone in masses ; matter of taste is the color or tone ; con- 

 ventionally fixed is the size' and "sharpness" goes without 

 saying. 



It is necessary that the argentic deposit forming the image 

 be very fine, partaking more of the nature of a stain than of 

 a granular deposit. Makers of plates for slides are so well 

 aware of this, and the processes for producing suitable emul- 

 sion for slides are so little prone to yield coarse deposits, that 

 we need do no more than point out the desideratum of a fine 

 grained image. But on this account, if on no other, gelatine 

 bromide emulsion such as is used for negative production, is 

 as a rule, totally unsuited for slide making purposes. 



" Wet collodion " stands pre-eminent among processes for 

 the production of photo-micrographic slides; on that point we 

 have no doubt whatever. Perfect clearness of lights, complete 

 opacity of lines when desired, sharpness, fineness of deposit, 

 half tone in masses all are obtained by the wet collodion 

 process with a very reasonable exposure in the camera; for 

 contact printing this process is not so convenient. But in any 

 case camera copying is preferable to contact printing for our 

 special purpose, even if our negative is the same size as our 

 slide is to be. In landscape and portrait slides a warm tone is 

 a very important feature, in the opinion of advanced photog- 



