SIMPLE MICROSCOPE. WOLLASTOX's DOUBLET. 21 



Sir D. Brewster ; who proposed to substitute diamond, sapphire, 

 garnet, 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 mag- 

 nifying power ; and the aperture would admit of great extension, 

 without a proportional increase in the spherical and chromatic 

 aberrations. This suggestion was carried into practice by Mr. 

 Pritchard with complete success, as regards the performance of 

 lenses executed on this plan ; but independently of the costliness 

 of their material, 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 have latterly been quite superseded by it.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 combination, first 

 proposed by Dr. Wollaston, 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. Wollaston's original combination, no perforated 

 diaphragm (or 'stop ') was interposed ; and the distance between 

 the lenses was left to be determined by experiment 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 between 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 spherical and the chromatic aberrations 

 are so much reduced, that the angle of aperture may be consider- 

 ably 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 microscope, 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 instrument would ever use the higher powers of the 

 former. It is for the prosecution of observations and for the 

 carrying on of dissections which only require low powers, that the 

 Simple microscope is to be preferred ; and consequently, although 

 doublets and triplets afforded the best means of obtaining a high 

 magnifying power, before Achromatic lenses were brought to their 

 present perfection, they are now comparatively little employed, 

 except as finders (§ 35.) 



19. Another form of Simple magnifier, possessing certain advan- 

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

 known by the name of the ' Coddington' lens.f The first idea of 



* "Transactions of the Society of Arts," Vol. xlix. 



t This name, however, is most inappropriate ; since Mr. Coddington 

 neither was, nor ever claimed to be, the inventor of the mode of con- 

 struction by which this lens is distinguished. 



