54 SELECTION AND USE 



made, even trifling delays must be considered. Some makers 

 have endeavored to avoid this difficulty by supplying double 

 fronts (a wet and a dry) to their objectives, while Mr. "Wenham 

 has elaborated a formula for a series of objectives which work 

 either wet or dry, according as the arrangement used for the 

 ordinary cover-adjustment is set to the one or the other. Not 

 having had an opportunity to examine objectives constructed 

 according to this plan, we cannot speak in regard to its success. 

 The origin of the immersion objective seems to be disputed. 

 So far as we can learn, the principle was first suggested by Sir 

 David Brewster, but the first really useful lenses of this kind 

 were brought out by Hartnack. 



Lens Systems. Formerly the term "system" was ap- 

 plied only to the entire combination forming the objective, and 

 we had "immersion systems," "correction systems," etc. At 

 present the word is used also to denote the individual combina- 

 tions of two or more pieces of glass, which, when arranged 

 together, form the whole objective, as will be understood from 

 Fig. 4, where 1, 2 and 3 form the separate systems, each com- 

 posed of two pieces of glass. Such a combination (the figure of 

 which is, of course, only diagramatic,) is said to form a three- 

 system lens. Very low powers, formed of two achromatic lenses, 

 are said to be two-system; four combinations, four-system, etc. 



French Triplets. A few years ago these objectives were 

 used quite extensively. They are so called because they ori- 

 ginated in the country after which they are named, while to 

 further distinguish between them and objectives constructed 

 according to the principles laid down by Lister, the latter were 

 known as the English form. Good makers of the English form 

 are now found in the United States, France, Germany, Austria 

 and Italy; and the French pattern is made in many of the 

 cities of Europe outside of France, although as yet neither the 

 English nor the American opticians have been able to manu- 

 facture them at prices which can compete with those of 

 continental Europe. The best of the so-called French ob- 

 jectives consist simply of lenses in which the chromatic aber- 

 ration is corrected by the usual plan of making each lens 



