84 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



produce a picture free from false color. If the object-glass had 

 been rendered perfectly achromatic, the blue rays, after passing 

 through the field-glass, would have been brought to a focus at 5, and 

 the red at r ; so that an error would be produced, which would have 

 been increased instead of antagonized by the eye-glass. Another 

 advantage of a well-constructed Huyghenian eye-piece is that 

 the image produced by the meeting of the rays after passing 

 through the field-glass, is by it rendered concave towards the 

 eye-glass, instead of convex, so that every part of it may be in 

 focus at the same time, and the field of view thereby rendered flat. 1 



22. Two or more Huyghenian eye-pieces, of different magni- 

 fying powers, known as Nos. 1, 2, 3, &c., are usually supplied 

 with a Compound Microscope. The utility of the higher powers 

 will mainly depend upon the excellence of the objectives; for 

 when an achromatic combination of small aperture, which is 

 sufficiently well corrected to perform very tolerably with a low 

 eye-piece, is used with an eye-piece of higher magnifying power 

 (commonly spoken of as a "deeper" one), the image may lose 

 more in brightness and in definition, than is gained by its ampli- 

 fication; whilst the image given by an objective of large angular 

 aperture and very 'perfect corrections, shall sustain so little loss 

 of light or of definition by "deep eye-piecing," that the increase 

 of magnifying power shall be almost all clear gain. Such an 

 objective, therefore, though of far inferior power in itself, is 

 practically more valuable (as giving a much greater range of 

 power with equal efficiency) than a lens of higher power which 

 can only be used effectively with the shallower eye-pieces. Hence 

 the mode in which different achromatic combinations of the same 

 power, whose performance with shallow eye-pieces is nearly the 

 same, are respectively affected by deep eye-pieces, afford a good test 

 of their respective merits ; since any defect in the corrections is 

 sure to be brought out by the higher amplification of the image, 

 whilst a deficiency of aperture is manifested by the want of light. 



23. An Eye-piece is sometimes furnished with achromatic 

 microscopes, especially for micrometric purposes, which, though 

 composed of only two plano-convex lenses, differs essentially in 

 its construction from the Huyghenian ; the field-glass having its 

 convex side upwards, and being so much nearer to the eye-glass, 

 that the image is not formed above it (as at B B, Fig. 12), but 

 below it. This eye-piece, which is known as Ramsden's, gives a 

 very distinct view in the central portion of the field; but, as it 

 does not, like the Huyghenian, correct the convexity of the image 

 formed by the object-glass, but rather increases it, the marginal 

 portions of the field of view, when the centre is in focus, are quite 

 indistinct. Hence this eye-piece cannot be recommended for 

 ordinary use ; and its chief value to the Microscopist has resulted 



1 Those who desire to gain more information upon this subject than they can from the 

 above notice of it, may be referred to Mr. Varley's investigation of the properties of the 

 Huyghenian eye-piece, in the 51st volume of the '"Transactions of the Society of Arts;" 

 and to the article " Microscope," by Mr. Ross, in the " Penny Cyclopaedia." 



