THE MICROSCOPE AND ITS KEVELATIONS. 



ence in the place of these images, their rays, after refraction by it, enter 

 the eye in a parallel direction, and produce a picture free from false color, 

 [f 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 , and the red at r; so that an error would be produced, which would 

 have been increased instead of being corrected by the eye-glass. Another 



advantage of a well-constructed Huyghen- 

 ian 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 Two or 

 more Huyghenian Eye-pieces, of differ- 

 ent magnifying powers, known as A, B, 

 C, etc., are usually supplied with a Com- 

 pound 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 cor- 

 rected to perform very tolerably with a 



' low ' or ' shallow ' eye-piece, is used with 



an eye-piece of higher magnifying power 



Section of Huyqhenian Eye-piece , J i & < < * J & f \ 



adapted to over-corrected Achroma- (commonly spoken of as a ' deeper ' one), 



the image may lose more in brightness and 



in definition than is gained by its amplification; whilst the image given by 

 an Objective of large angular aperture and very perfect correction, shall 

 sustain so little loss of light or of definition by ' deep eye-piecing/ that the 

 increase of magnifying power shall be almost clear gain. Hence the modes 

 in which different Objectives 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. The working Microscopist will generally find the A eye-piece 

 most suitable, B being occasionally employed when a greater power is 

 required to separate details, whilst C and other still deeper are useful for 

 the purpose of testing the goodness of Objectives, or for special investi- 

 gations requiring the highest amplification with Objectives of the finest 

 quality. When great penetration or ' focal depth' is required, low Ob- 

 jectives and deep Eye-pieces will often be found convenient. 



28. For viewing large flat objects, such as transverse sections of 

 Wood (Chap, ix.) or of Echinus-spines (Plate n. Fig. 1), under lov> r 

 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 interposi- 

 tion of a diaphragm; and the eye-glass is an achromatic combination of a 



1 Those who desire *o 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 th& 

 " Penny Cyclopaedia," reprinted, with additions, in the " English Cyclopaedia." 



