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POPULAR SCIENCE MONTHLY. 



excellent results are obtained. It is a singular fact that almost every 

 snow crystal differs in some particular from all others collected. Mr. 

 Bentley, of Nashville, Vermont, has doubtless given more attention to 

 snow crystal photography than any one else, and the number of different 

 forms which he has already obtained is remarkably large, and many of 

 them are exquisite. Much has also been done on questions relative to 

 the crystalline structure of various minerals by the aid of the micro- 

 scope, as for example, 'Inclusions of Petroleum in Quartz' (Jour. Am. 

 Chem. Soc.) 20: 795, and also concerning the 'Solution Vein Theory' 

 for the origin of gold. In the same way it is possible that in the 

 future a unanimity of opinion may be produced concerning the crystal- 

 lization of iron, which is worthy of serious attention as the enormous 

 amount of that metal used in construction increases year by year. 



Fig. 20. Cow Hair, 

 eters 



2,200 Diam- Fig. 21. Cross Section of Horn of Afri- 



can Rhinoceros, x 60 Diameters. Polar- 

 ized Light. 



The value of the part which the microscope plays in determining 

 facts about the minute structure of the animal organisms can scarcely 

 be overestimated. The bacteriological analysis of water in connection 

 with zymotic diseases; the determination of disease germs in impure 

 air; the examination of useful and injurious bacteria in food; all this 

 and more must be credited to the microscope in the development of 

 medical science and sanitation. 



A detailed description of all the applications of photomicrography to 

 nature study can not well be given here, and so a few of the important 

 ones only have been briefly noticed. The chief value of it all seems to 

 be the aid rendered in the dissemination of knowledge. To attempt to 

 look beneath the superficial and discover the ulterior is a fundamental 

 desire in the active, civilized mind. Since the beginning of the seven- 

 teenth century, when Galileo was imprisoned for stating his belief in 

 the motion of the earth, we look with pride at the development of science 



