45 



that they give a better illuminated field, as there is 

 less loss of light by absorption through the glass than 

 by refraction at the two additional surfaces of the eye- 

 lens and field-lens in the Huyghenian. 



Periscopic Eye-Piece. This consists of a triple 

 eye-lens and single field-lens. Its predominant fea- 

 ture is a very large and flat field, 

 with almost all objectives. In 

 this respect it has a considerable 

 advantage over the Huyghenian 

 and Solid. It is positive and 

 therefore well adaped for micro- 

 meter work, as it is focused like 

 a magnifier, and its magnifying 

 power remains constant, while 

 with the Huyghenian it is vari- 

 Fig. 14. able, from the fact that the eye- 



lens alone is focused, thus varying its distance from 

 the field-lens, and consequently the magnifying power. 

 Nomenclature. The rating of eye-pieces was 

 formerly, and is to a considerable extent to-day, by 

 letters. This method, however, is arbitrary, as the 

 letters of different makers have a totally different sig- 

 nificance, so that nothing like a standard exists. This 

 fact induced the American Society of Microscopists 

 to endeavor to establish a universal method, and after 

 the matter had been given careful attention for sev- 

 eral successive years, it finally adoped the method 

 which rates them according to their magnifying 

 powers, the same as that which has been used in 

 objectives. .This gives an approximate idea of the 



