26 OPTICAL PEINCIPLES OF THE MICROSCOPE. 



shorter for blue rays than for red rays by just the difference 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 

 colour. 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 b, 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 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.* — Two or more Huyghenian 

 Eye-pieces, of different magnifying powers, known as A, B, C, &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 

 amplification ; 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 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 the most suitable, B being 

 occasionally employed when a greater power is required to separate 

 details, whilst C and others still deeper are chiefly useful for the 

 purpose of testing the goodness of Objectives. 



22. An Eye-piece is sometimes furnished with Achromatic Micro- 

 scopes, especially for micrometric purposes, which, though com- 

 posed of only two plano-convex lenses, differs essentially in its 

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

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

 the image formed by the object-glass does not lie above (as at b b, 

 Fig. 14), but below it. This ' positive' eye-piece, which is known 

 as Ramsde7i's, gives a very distinct view in the central portion of 



* 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," reprinted, with 

 additions, in the "English Cyclopaedia." 



