ELEMENTARY PHOTO-MICROGRAPHY. 21 



microscopist cannot be free from the " personal 

 equation," which plays so important a part in 

 the registration of scientific observations, and is 

 one of the most fruitful sources of error in such 

 work. Another authority, Cole, also names the 

 imperfections of drawings, however clever, whilst 

 claiming for the photo-micrograph freedom from 

 all defects, colour alone excepted. The dominating 

 element of accuracy should be borne in mind by 

 all photo-micrographers. 



Some years ago, even in our large cities, there 

 were few persons capable of producing good photo- 

 micrographs, while to-day they are produced in 

 every branch of science. 



In 1891 I had occasion to require some photo- 

 micrographs of sand, but, to my surprise, I was 

 informed that in a city of 400,000 inhabitants 

 only two or three persons outside the Medical 

 School were competent to do the work. 



There is no reason why the art should be con- 

 fined to the select few, for, after all, the mysteries 

 are not so great that they may not be lessened by 

 a few preliminary trials, one or two failures being 

 sufficient to indicate the direction in which an 

 alteration should .be made. The apparatus need 

 not be at all costly in order to produce good 

 results, although for high-class scientific research 

 everything must necessarily be of the best. 



