MICROSCOPE. 



Go 5 



have. The advantages of this construction may 

 be regarded as similar to those of the grooved 

 sphere. 



The Periscopic Achromatic Sphere This is a 

 third suggestion by Sir D. Brewster for the con- 

 struction of a microscopic sphere. Referring to 

 the diagram ; A B and C D 

 are two double convex lenses, 

 having their outer surfaces 

 portions of the same sphere ; 

 they are united at the centre, 

 as in the case of the perisco- 

 pic sphere, but the fluid oc- 

 cupying the space g h t k, in- 

 stead of being of equal refrac- 

 tive power with the lenses, is to be adapted for 

 the correction of chromatic aberration. A convex 

 speculum e g h f is cemented on the lens C D 

 for illuminating the object ; and the aperture g h 

 cuts off the oblique rays that would otherwise 

 confuse the image and contract the available field 

 of view. We presume this sphere must be very 

 perfect in its definition, as it combines the ad- 

 vantages of the grooved sphere, with an entire 

 correction of the chromatic aberration. The mag- 

 nifying power is that of a sphere, diminished by 

 a double concave lens, equal to ef A B. 



LENSES. The various figures and refractive pro- 

 perties of lenses are fully explained and illustrated 

 underOptics (qu. vide). The inconveniences attend- 

 ing the use of the small spherule, led to the re- 

 adoption of the double convex lens as a more satis- 

 factory magnifying power, for it was perceived 

 that what it lost in the amplification of the image, 

 was more than compensated for by the increase of 

 light, and the much greater extent of the field of 

 view. The performance of double convex lenses 

 of deep power, though more perfect than that of 

 the spherule, was however less satisfactory than 

 could be wished, the spherical and chromatic aber- 

 rations being very considerable. The plane convex 

 lens was then introduced into the microscope; but 

 as this magnifies only half so much as a double 

 convex lens of equal convexity, it necessarily 

 formed, when adopted as a deep magnifier, a por- 

 tion of a very minute sphere. The exposure of 

 its plane surface to the object, corrected in a con- 

 siderable degree the unequal refraction of the 

 oblique rays, and gave breadth to the field ; but 

 from the extremely small size of the lens, the light 

 was too feeble to render its use pleasant. In these 

 observations we shall be understood to refer to the 

 properties of the convex lens per se ; for it is 

 scarcely necessary to remark, that in the compound 

 instruments successively invented, the imperfect 

 action of the uncombined lens was to a certain 

 extent overcome. We now come to the consider- 

 ation of the recent improvements in these simple 

 magnifiers. 



The superiority of the best convex lenses now 

 in use, over those to which we have adverted, re- 

 sults partly from the ratio of their curved surfaces, 

 and principally from the qualities of the diapha- 

 nous substances employed in their formation. 

 Glass has a very low refractive power ; and this 

 circumstance rendered it desirable that some more 

 suitable transparent medium should if possible be 

 found. The attention of the scientific world was 

 first called to this subject by Sir D. Brewster, who 

 not only clearly defined the desiderata in micro- 

 scopic elements, but experimented most, successfully 

 with gems, and thus led the way to all the im- 



provements in the lens, resulting from the quality 

 of its substance. 



The Diamond Lens. Attention having been 

 awakened to the highly refractive powers of gems, 

 and their consequent fitness for magnifiers, Dr 

 Goring and Mr Pritchard were led to experiment 

 on the diamond ; and after a course of persevering 

 and ingenious trials, the latter gentleman succeeded 

 in bringing this precious substance to the spherical 

 figure, and rendering its action satisfactory. Now 

 to appreciate duly the value of a diamond lens, we 

 must bear in mind that the thinner a lens is, or, 

 in other words, the slighter the convexity of its 

 sides may be, the more perfect will be its action as 

 a magnifying power. For if a lens be too thick 

 many rays are dissipated in their passage, and the 

 divergent ones are refracted very unequally ; arid 

 when the lens is a portion of a small sphere, it is 

 found that diminution of aperture, and consequent 

 loss of light, are a resulting inconvenience. Now a 

 diamond lens, the radius of whose convexity is 8, 

 has equal refractive power with a glass lens curved 

 to a radius of 3 ; hence, we can either obtain much 

 deeper magnifiers with the diamond, or gain vast 

 advantages from the diminution of central thick- 

 ness, and the greater radius of the lens. It is only 

 necessary to add that a sphere or spherule of 

 diamond would be. totally useless; the highly re- 

 fractive power causing the focus to fall beneath 

 the surface. 



The Sapphire Lens. The sapphire has a much 

 lower refractive power than the diamond, its radius 

 of convexity being to that of glass for the same 

 magnifying power, as 5 is to 3. A lens of this 

 substance is injured also, more or less, by double 

 refraction, which duplicates and confuses the de- 

 tails of an image. Methods are in use for counter- 

 acting this inconvenience to a certain extent ; but 

 it would appear that the sapphire is better calcu- 

 lated to give the benefits of large aperture arid 

 increased light, than the advantages of a deep 

 magnifying power. 



The Garnet Lens. The colour of this gem 

 would on first consideration appear to render it an 

 unsuitable substance for lenses ; yet no great in- 

 convenience is found to result from it, as the tinge 

 is exceedingly slight when the lens is of moderate 

 thickness. The index of refraction in the sapphire 

 andgametisthesame; thelatterishoweverfree from 

 the disadvantages of double refraction, and experi- 

 ence decides that it is altogether the most pleasant 

 and useful medium of which lenses can be formed, 

 of course excepting the diamond, whose costliness 

 renders it comparatively inaccessible. 



The Cata-dioptric Lens. This is a contrivance 

 by Sir D. Brewster for obtaining from ahemisplieri- 



cal plano-convex lens a double power, through the 

 joint agency of refi action and reflection. 



