CONSTRUCTION OF EYB-PIBCES. 85 



from its adaptation to receive a divided glass micrometer, which 

 may be fitted into the exact plane wherein the image is formed 

 by the object-glass, so that its scale and that image are both 

 magnified together by the lenses interposed between them and 

 the eye. We shall hereafter see, however, that the same end 

 may be so readily attained with the Huyghenian eye-piece (§ 46), 

 that no practical advantage is gained by the use of that of Eams- 

 den. It is affirmed by Mr. Eoss, that if the Achromatic princi- 

 ple were applied to the construction of Eye-pieces, the latter is 

 the form with which the greatest perfection would be obtained. 

 That such an adaptation might be productive of valuable results, 

 appears from the success with which Mr. Brooke has employed 

 a triplet objective of one-inch focus, as an eye-piece; the defini- 

 tion obtained by it being very superior to that afforded by the 

 ordinary Huyghenian eye-piece. 



24. In the Eye-pieces of compound microscopes of older con- 

 struction, it was customaiy to employ a pair of piano or double- 

 convex lenses of longer focus, for the eye-glass, instead of a 

 single plano-convex of shorter focus ; the advantage being, that a 

 larger and flatter field could be thereby obtained. A brighter 

 image, a fiatter field, and a greater freedom from aberration, than 

 are afforded by any ordinary eye-piece of this kind, may be ob- 

 tained by the substitution of a combination nearly resembling 

 Herschel's "aplanatic doublet" — namely,' a meniscus, having its 

 concave side next the eye, and a double-convex of the form of 

 least aberration," with its flattest side next the object — for the 

 plano-convex eye-glass; and the substitution of a double-con- 

 vex lens of the form of least aberration, with its flattest side 

 next the object, for the plano-convex fleld-glass. With such 

 an eye-piece, a field of fourteen inches in diameter (measured at 

 the usual distance of ten inches) may be obtained perfectly flat, 

 and equally distinct and well illuminated over every part. When 

 such an eye-piece, however, is used in conjunction with achroma- 

 tic objectives, it impairs the definition of their image to such a 

 degree, that their finest qualities are altogether sacrificed. Still 

 there are certain large transparent objects, such as transverse 

 sections of wood, wings of insects, &c., in viewing which a large 

 and flat field is of more importance than perfect definition; since 

 their structure is so coarse, that there is no minute detail to be 

 brought out. I^othing is so effective for the exhibition of these, 

 as an eye-piece of the kind just alluded to, with an objective of 

 about 3 or 4 inches focus ; and this may either be a single lens 

 (which, when of ^uch low power, will perform sufficiently well for 

 objects of this class), or a single pair of lenses forming part of a 

 perfect achromatic combination, having its aperture somewhat 

 contracted by a stop.' 



' The " fcrm of least aberration" is when the radii of the two surfaces are to each other 

 as 1 to 6. 



^ Some of the lowest French Aohromatics answer extremely well for this purpose ; and 

 the front pair of the lowest set usually made in this country (that, namely, of 2 inches 

 focus) is sometimes made removable, so that the back pair, which also is very suitable to 

 the class of objects mentioned above, may be employed by itself. 



