OPTICAL PRINCIPLES OF THE MICROSCOPE. 25- 



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 Ob- 

 jectives for use with Huyghenian eye-pieces ( 27). A flat well-illumi- 

 nated field of as much as fourteen inches in diameter may thus be ob- 

 tained 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 sur- 

 face of the field- glass is made so unpleasantly obvious, that the most care- 

 ful 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 for the ordinary wants of the working 

 Microscopist. 



29. A solid Eye-piece made on the principle of the ( Stanhope' lens 

 ( 24) is sometimes used in place of the ordinary Huyghenian, when high 

 magnifying power is required for testing the performance of Objectives. 

 The lower surface, which has the lesser convexity, serves as a 'field-glass;' 

 whilst the image formed by this is magnified by the highly convex upper 

 surface to which the eye is applied; the advantage supposed to be derived 

 from this construction lying in the abolition of the plane surfaces of 

 the two lenses of the ordinary eye-piece. A < positive ' or Ramsden's Eye- 

 piece i n which the field glass, whose convex side is turned upwards, is 

 placed so much nearer the eye-glass that the image formed by the Objec- 

 tive lies below instead of above it, was formerly used for the purpose of 

 Micrometry; a divided glass being fitted in the exact plane occupied by 

 the image, so that its scale and that image are both magnified together 

 by the lenses interposed between them and the eye. The same end, how- 

 ever, may be so readily attained with the Huyghenian eye-piece ( 91), 

 that no essential advantage is gained by the use of that of Kamsden, the 

 field of which is distinct only in its centre. 



4. Stereoscopic Binocular Microscope. 



30. The admirable invention of the Stereoscope by Professor Wheat- 

 stone, 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 Steroscope 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 ta 

 work with the Stereoscopic Binocular 1 upon objects that are peculiarly 

 adapted to its powers. As the result of this application cannot be rightly 

 understood without some knowledge of one of the fundamental principles 

 of Binocular vision, a brief account of this will be here introduced. All 

 vision depends in the first instance on the formation of a picture of the 

 object upon the retina of the Eye, just as the Camera Obscura forms a 

 picture upon the ground glass placed in the focus of its lens. But the 

 two images that are formed by the two eyes respectively, of any solid 

 object that is placed at no great distance in" front of them, are far from 



1 It has become necessary to distinguish the Binocular Microscope which gives 

 true Stereoscopic effects by the combination of two dissimilar picture, from a 

 Binocular which simply enables us to look with both eyes at images which are- 

 essentially identical (81). 



