80 OPTICAL PEINCIPLES OF THE MICROSCOPE. 



and other precious stones of high refractive power, for glass, as 

 the material of single lenses. A lens of much longer radius of 

 curvature might thus be employed, to gain an equal magnifying 

 power ; and the aperture would admit of great extension, with- 

 out a proportional increase in the spherical and chromatic aber- 

 rations. This suggestion has been carried into practice with 

 complete success, as regards the performance of lenses executed 

 on this plan ; but the difficulties of various kinds in the way of 

 their execution, are such as to render them very expensive ; and 

 as they are not superior to the combination now to be described, 

 they have latterly been quite superseded by it. This combina- 

 tion, first proposed by Dr. WoUaston, and known as his doublet, 

 consists of two plano-convex lenses, whose focal lengths are in 

 the proportion of one to three, or nearly so, having their convex 

 sides directed towards the eye, and the lens of shortest focal 

 length nearest the object. In Dr. WoUaston's original combina- 

 tion, no perforated diaphragm (or " stop") was interposed; and 

 the distances between the lenses was left to be determined by ex- 

 periment in each case. A great improvement was subsequently 

 made, however, by the introduction of a " stop" between the 

 lenses, and by the division of the power of the smaller lens be- 

 tween two (especially when a very short focus is required) so as 

 to form a triplet, as was first suggested by Mr. Holland.* When 

 combinations of this kind are well constructed, both the spheri- 

 cal and the chromatic aberrations are so much reduced, that the 

 angle of aperture may be considerably enlarged without much 

 sacrifice of distinctness ; and hence for all powers above l-4th 

 inch focus, doublets and triplets are far superior to single lenses. 

 The performance of even the best of these forms of Simple mi- 

 croscope, however, is so far inferior to that of a good Compound 

 microscope as now constructed upon the achromatic principle, 

 that no one who has the command of the latter form of instru- 

 ment would ever use the higher powers of the former. It is for 

 the prosecution of observations, and for the carrying on of dis- 

 sections, which only require low powers, that the Simple micro- 

 scope is to be preferred ; and, consequently, although doublets 

 and triplets aflbrded the best means of obtaining a high magni- 

 fying power, before Achromatic lenses were brought to their pre- 

 sent perfection, they are now comparatively little used. 



19. Another fonn of simple magnifier, possessing certain ad- 

 vantages over the ordinary double-convex lens, is that commonly 

 known by the name of the " Coddington" lens. The first idea 

 of it was given by Dr. WoUaston, who proposed to cement to- 

 gether two plano-convex, or hemispherical lenses, by their plane 

 sides, with a stop interposed, the central aperture of which should 

 be equal to l-5th of the focal length. The great advantage of 

 such a lens is, that the oblique pencils pass, like the centre ones, 

 at right angles with the surface ; and that they are consequently 

 but little subject to aberration. The idea was further improved 



' "Transactions of the Society of Arts," vol. xlix. 



