80 SELECTION AND USE 



must be considered. Some makers have endeavored to avoid 

 this difficulty by supplying double fronts (a wet and a dry) to 

 their objectives, while others have constructed 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. 



Lens Systems Duplex Fronts. Formerly the term 

 "system" was applied 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 combinations of two or more pieces of glass, which, 

 when arranged together, form the whole objective, as will be 

 understood from Fig. 16, where 1, 2 and 3 form the separate 

 systems, each composed of two pieces of glass. Such a com- 

 bination (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 com- 

 binations, four-system, etc. 



Some years ago, Mr. Tolles brought out objectives of remark- 

 able power, which were known as "duplex fronts," or "four- 

 system objectives. These objectives were so called because a 

 second lens (nearly hemispherical) was added to the front com- 

 bination, and this additional lens, added to the three already 

 in use, made a four-system arrangement. 



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 



