84 ELEMENTARY PHOTO-MICROGRAPHY. 



Slides deep yellow on one side and perfectly trans- 

 parent on the other, whilst not unfit for visual 

 examination, are quite unsuitable for photography. 

 This shows that some attention ought to be paid 

 to the quality of the mounting. 



Again, the kind of medium in which the object 

 is mounted will largely determine the character 

 of the photograph, more particularly with high 

 powers. Many varieties of preservative, resinous 

 and chemical media are employed for slide mount- 

 ing, and advantage can be taken of such as have 

 a high refractive index to bring out distinctly all 

 possible details. 



Van Heurck says that any object inserted in 

 a liquid is visible according to the amount of 

 difference between the refractive indices of the 

 medium and the object ; and by way of example 

 he compares a diatom mounted in water with one 

 mounted in styrax, and also in monobromide of 

 naphthalin. 



The silica of diatoms has a refractive index of 

 i 43, and the index of water is 1.33, so that the 

 difference, viz., 10, expresses the visibility in 

 water. With styrax this difference is 17, and 

 with monobromide 22. With realgar a much 

 greater difference can be obtained ; but unfor- 

 tunately good mounts in this medium are difficult 

 to obtain. 



