16 THE MICROSCOPE AND ITS REVELATIONS. 



has shown l that the whole color-correction may be effected in the mid- 

 dle lens by a double-concave of dense flint between two convex lenses of 

 crown, the back lens as well as the front being a single plano-convex of 

 crown. Thus one double concave lens of flint is made to correct the 

 chromatic aberrations of four convex surfaces of crown, the total number 

 of surfaces being reduced to ten. There is a further advantage in this 

 plan of construction, that no change of the front lens is needed to enable 

 the combination to be used as an ' immersion ' objective ( 19), the requi- 

 site adjustment being effected by the screw-collar used for cover-correc- 

 tion. There can be no doubt that objectives of moderate angular aperture 

 may be made on Mr. Wenham's system, so as to combine great excellence 

 with comparative cheapness; but it does not seem equally suitable for 

 first-class objectives, requiring for their greatest efficiency the widest 

 attainable angular aperture. These have usually been made to consist of 

 a front triplet, a middle doublet, and a back triplet, thus having eight 

 lenses in all, with sixteen surfaces. But the first-class constructors in 

 the United States (notably Messrs. Tolles, Spencer, and Wales) have 

 added to these a single front plano-convex of crown, by means of which 

 a longer working distance has been obtained; whilst the extraordinary 

 excellence of their workmanship (only -attain able, however, at a very high 

 cost) has given to these very complex combinations a perfection of per- 

 formance, which, to say the least, is unsurpassed by that of any objectives 

 constructed for use in the ordinary manner, which is now distinguished 

 as dry. 



19. It was long since pointed out by Amici that the introduction of a 

 drop of water between the front surface of the objective, and either the 

 object itself or its covering-glass, would diminish the loss of light result- 

 ing from the passage of the rays from the object or its covering-glass into 

 air, and then from air into the object-glass. But it is obvious that when 

 the rays enter the object-glass from water, instead of from air, both its 

 refractive and its dispersive action will be greatly changed, so as to need 

 an important constructive modification to suit the new condition. This 

 modification seems never to have been successfully effected by Amici 

 himself; and his idea remained unfruitful until it was taken up by Hart- 

 nack and Nachet, who showed that the application of what is now known 

 as the Immersion- system to objectives of high power and large angular 

 aperture is attended with many advantages not otherwise attainable. 

 For, as already pointed out ( 1), the loss of light increases with the 

 obliquity of the incident rays; so that when objectives of very wide angle 

 of aperture are used ' dry/ the advantages of its increase are in great 

 degree nullified by the reflection of a large proportion of the rays falling 

 very obliqely'upon the peripheral portion of the front lens. When, on 

 the other hand, rays of the same obliquilyenter the peripheral portion of 

 the lens from water, the loss by reflection is greatly reduced, and the ben- 

 efit derivable from the large aperture is proportionally augmented. 

 Again, the ' immersion system' allows of a greater working distance 

 between the objective and the object, than is otherwise attainable with 

 the same extent of angular aperture; and this is a great advantage, not 

 merely in regard to convenience in manipulation, but also in giving a 

 greater range of * penetration ' or ' focal depth.' Further, the observer 

 is rendered less dependent upon the exactness in the correction for the 

 thickness of the covering-glass, which is needed where objectives of large 



1 " Proceedings of Royal Society," Vol. xxi., p. 111. 



