THE SELECTION OF OBJECTIVES. 49 



No objective with an air angle of more than 40 should 

 be used with the Wenham binocular. Dr. Carpenter 

 pointed out long ago the exaggerated effect of projection 

 produced when pollen-grains of the Malvacece and other 

 similar objects are examined binocularly with high-angle 

 objectives ; perfectly spherical objects, instead of resembling 

 a hemisphere, appearing like the small end of an egg. 



Powers yielding amplifications ranging between 50 and 

 200 diameters may be called medium, and are represented 

 in our list by the J-inch and J-inch objectives. The former 

 is a very handy glass, though it does not seem to be an 

 easy one to construct, judging from many the author has 

 seen. Makers of cheap but really good i-inch and J-inch 

 powers seem to fail sometimes in producing cheap and 

 perfect J-inch objectives. 



A good working J-inch may have an aperture of 60, 

 though it is made of 35 by Mr. Browning, and 40 by 

 Messrs. Powell and Lealand and Mr. Collins, for special 

 use with the binocular. 



The medium and higher powers should be chosen with 

 conical fronts, as shown in Fig. 30, for with them it is 

 more easy to illuminate opaque objects satisfactorily. The 

 illustration is that of Mr. Swift's short mounted -inch 

 objective, with collar adjustment. 



Zentmayer, Wray, and Zeiss have produced low power 

 objectives in which the two combinations composing them 

 are separated or brought nearer to each other by means 

 of a screw collar, the lens being nominally a 4-inch, a 2-inch, 

 or any intermediate power at will. This glass defines well, 

 and, moreover, possesses a flat field ; but, according to 

 measurement, the amplification it yields more nearly 

 approaches a 5-inch and 3-inch than an English 4-inch 

 and 2-inch. 



For several years cheap foreign lenses -were in great 



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