bamsden's and kellner's eye-pieces. 27 



the field ; but, as it does not, like the Huyghenian, correct the 

 convexity of the image formed by the object-glass, but rather in- 

 creases it, the marginal portions of the field of view, when the 

 centre is in focus, are quite indistinct. Hence this Eye- piece can- 

 not be recommended for ordinary use ; and its chief value to the 

 Microscopist has resulted 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 

 (§ 68), that no essential advantage is gained by the use of that of 

 Rarasden. — For viewing large flat objects, such as transverse sections 

 of Wood (Plate xu.) or of Echinus-spines (Plate n. Fig. 1), under 

 low magnifying powers, the Eye-piece known as Kellner's may be 

 employed with advantage. In this construction the Field-glass, 

 which is a double- convex lens, is placed in the focus of the Eye- 

 glass, without the interposition of a diaphragm ; and the Eye-glass 

 is an achromatic combination of a plano-concave of flint with a 

 double-convex of crown, which is slightly under-corrected, so as to 

 neutralize the over- correction given to the Objectives that are 

 ordinarily used with Huyghenian eye-pieces (§ 21). A flat well- 

 illuminated field of as much as fourteen inches in diameter may 

 thus be obtained with very little loss of light ; but, on the other 

 hand, there is a certain impairment of defining power, which 

 renders the Kellner eye-piece unsuitable for objects presenting 

 minute structural details ; and it is an additional objection that 

 the smallest speck or smear upon the surface of the field- glass is 

 made so unpleasantly obvious, that the most careful cleansing of 

 that surface is required every time that this Eye-piece is used. 

 Hence it is better fitted for the occasional display of objects of the 

 character already specified, than it is for the ordinary wants of 

 the working Microscopist. 



4. Stereoscopic Binocular Microscope. 

 23. The admirable invention of the Stereoscope by Professor 

 Wheatstone, has led to a general appreciation of the value of the 

 conjoint use of both eyes in conveying to the mind a notion of the 

 solid forms of objects, such as the use of either eye singly does 

 not generate with the like certainty or effectiveness. And after 

 several attempts, which were attended with various degrees of 

 success, the principle of the Stereoscope has now been applied to 

 the Microscope, with an advantage which those only can truly 

 estimate, who (like the Author) have been for some time accustomed 

 to work with the Stereoscopic Binocular* upon objects that are 



* It has become necessary to distinguish the Binocular Microscope 

 which gives true Stereoscopic effects by the combination of two dissimilar 

 pictures, from a Binocular which simply enables us to look with both eyes 

 at images which are essentially identical (§ 62). 



