MICROSCOPE. 



341 



been proposed by different opticians, and one 

 or other will be preferred, according to the 

 purpose for which it be required. It may be 

 laid down as a general principle, however, that, 

 to give the highest effect to the microscope, in 

 regard to clearness of view and penetrating 

 power, no more than two lenses should be 

 employed; and that when a certain amount 

 of these may be sacrificed to gain a large flat 

 field, tliree is the largest number which can 

 be introduced with any benefit. This prin- 

 ciple is founded on the fact that, whenever 

 light impinges on the surface of even the most 

 transparent body, a part of it only is trans- 

 mitted, the remainder being reflected. In the 

 passage of light through ordinary lenses, there- 

 fore, a certain quantity is lost by reflection 

 at each surface; and every multiplication in 

 the number of lenses entails, therefore, a 

 positive evil, which may or may not be 

 counterbalanced by the good it effects. In 

 the doublet or triplet already described, 

 the correction of the aberrations is an advan- 

 tage much greater than the injury resulting 

 from the substitution of four or six surfaces for 

 two ; but this is by no means the case in the 

 eye-piece, in which (from their low power) the 

 aberrations are much less. Hence, when too 

 many lenses are employed in it, although the 

 field of view (that is, the circle within which 

 the image is comprehended) may be very much 

 enlarged and rendered flatter, the brilliancy 

 and sharpness of the image are so much im- 

 paired, and it is invested with so much false 

 colour, that, for all scientific purposes, the 

 instrument is rather deteriorated than im- 

 proved. 



The eye-piece which may be most advan- 

 tageously employed with achromatic object- 

 glasses, to the performance of which it is 

 desired to give the greatest possible effect in 

 regard to defining and penetrating power, with- 

 out the necessity of a large field, is that termed 

 the Iluyghenian, having been employed by 

 Huyghens for his telescopes, although without 

 the knowledge of all the advantages which its 

 best construction rendered it capable of afford- 

 ing. It consists of two plano-convex lenses 

 with their plane sides towards the eye. These 

 are placed at a distance equal to half the sum 

 of their focal lengths ; or, to speak with more 

 precision, at half the sum of the focal Ien2,lh 

 of the eye-glass, and of the distance from the 

 field-glass at which an image of the object- 

 glass would be formed by it. A stop or dia- 

 phragm must be placed half-way between the 

 two lenses. By Huyghens this arrangement 

 was intended 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. Since the 

 introduction of achromatic object-glasses for 

 compound microscopes, it has been further 

 shown that all error m;iy be avoided by a slight 

 over-correction of these, so that the blue and 

 red rays may be caused to enter the eye in a 

 parallel direction, and thus to produce a colour- 

 less image, though not actually coincident (Jig. 



Fig. 164. 



Section of Hni/ijJienian ei/e-piece, adapted to over- 

 corrected achromatic^. 



L M N, the two extreme rays of three pencils, 

 which, without the field-glass, would form a blue 

 image convex to the eye-glass at H 15, and a red 

 one at R R. By the field-glass, however, a 

 blue image, concave to the eye-glass, is formed 

 at B' B', and a red one at R' R'. As the focus 

 of the eye-glass is 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 tree from lalse colour. If the object- 

 glass had been rendered perfectly achromatic, 

 the blue rays, after passing 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 antago- 

 nised by the eye-glass. 



164). Further, the image produced by the 

 meeting of the rays after passing through the 

 field-glass is by it rendered convex towards the 

 eye-glass, instead of concave, so that every part 

 of it may be in focus at the same time, and the 

 field of view thereby rendered flat. Those who 

 desire to gain more information upon this sub- 

 ject than they can from the accompanying dia- 

 gram and the explanation of it, may be re- 

 ferred to Mr. Varley's investigation of the pro- 

 perties of the Huyghenian eye-piece in the 

 51st volume of the Transactions of the Society 

 of Arts, and to the article "Microscope" in 

 the Fenny Cyclopaedia. 



By an achromatic object-glass for a com- 

 pound microscope, therefore, is not meant one 

 which simply contains within itself a perfect 

 correction for its own errors, but one in which 

 the usual order of dispersion is so far reversed 

 that the light, after undergoing the series of 

 changes effected by the eye-piece, shall come 

 uncoloured to the eye. " We can give no spe- 

 cific rules," says the writer of the article just 



