42 VISION WITH THE COMPOUND MICROSCOPE 



towards the eye. A ' stop ' or diaphragm, B B, must be placed 

 between the two lenses, in the visual focus of the eye-glass, which 

 is, of course, the position wherein the image of the object will be 

 formed by the rays brought into convergence by their passage 

 through the field-glass. Huyghens devised this arrangement merely 

 to diminish the spherical aberration ; but it was subsequently shown 

 by Boscovich that the chromatic dispersion was also in great part 

 corrected by it. With the apochromatic lenses of the highest and 

 best quality (see Chapter Y.) no amount of obtainable eye-piecing, if 

 it be of the * compensation ' form, can break down the image. The 

 editor has tried in vain to break down the image formed by a 

 24 mm., a 12 mm., a 6 mm., and a 4 mm., all dry apochromatics 

 by Zeiss, and especially with a Jth by Powell and Lealand. It 

 is, however, a matter of moment and interest to note that with 

 good objectives of the ordinary achromatic construction of large 

 N.A. the compensating eye-pieces give better results than 

 Huyghenian. 



But of the old form of achromatic object-glass it is true of the 

 majority that they will not bear high eye-piecing. ' B,' 1 ^ inch in 

 focus, is a convenient and useful eye-piece for viewing large flat 

 objects, such as transverse sections of wood or of echinus-spines, 

 under low magnifying powers. A flat large field may be obtained 

 by means of a Kellner ; but, on the other hand, there is a very 

 serious falling off 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 unplea- 

 santly 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 for the scientific requirements of the working 

 microscopist. 



A ' positive ' or Ramsden's eye-piece in which the field-glass, 

 whose convex side is turned upwards, is placed so much nearer the 

 eye-glass that the image formed by the objective lies below instead 

 of above it is sometimes used for the purpose of micrometry, a 

 divided glass being fitted in the exact plane occupied by the image, 

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

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

 ever, is also attained with the Huyghenian eye-piece, and it is 

 doubtful if any advantage is gained by the Ramsden in microscope 

 work. The compensating eye-piece is also used in conjunction 

 with the micrometer. 



Aperture in microscopic objectives and the principles of micro- 

 scopic vision. It is now of the utmost moment that we should 

 understand clearly the meaning and importance of 'aperture' in 

 microscopic objectives, and by that means be led to a perception of 

 the principles of the most recent and only rational theory of micro- 

 scopic vision. Within the last twenty-five years this entire subject 

 has undergone a rigorous and exhaustive reinvestigation by one 

 of the most competent and masterly mathematical and practical 



