264 THE SIMPLE MICROSCOPE. 



greater distance of the object from the first refracting surface. In 

 order to further increase this distance with lower powers, such as 

 are employed in dissections upon the object-stage, he applied above 

 the doublet an achromatic concave lens, which not 

 only answers the purpose in view, but also somewhat 

 increases the amplification ; a contrivance which was 

 employed later on by Briicke also for his achromatic 

 magnifying-glass 1 (Fig. 148). 



The doublets of Chevalier, according to Harting's 

 observations, are about equal to those of Pritchard. 

 With an amplification of 48 linear, the lines of 

 the first band were resolved on Nobert's (30-band) 

 test-plate, and with 317 linear those of the sixth 

 band. 



Both Chevalier and Pritchard also made triplets, 

 the construction of which is based essentially on the same prin- 

 ciple. They consist of three plano-convex lenses of different focal 

 lengths, and if the necessary care is bestowed upon their manu- 

 facture they give images somewhat more distinct than the 

 doublets. 



It is unquestionable that Wollaston's idea of directing special 

 attention to the relative distances of the lenses, was a very happy 

 one for practical Optics, and led immediately to important improve- 

 ments. The doublet reached a degree of perfection which is still 

 sufficient, even at the present time, in many practical cases. But 

 the best combinations of this kind always suffer to a very consider- 

 able extent from faulty correction of spherical, and especially of 

 chromatic, aberration, 2 which is, of course, all the more palpable 

 the higher the amplification. As regards optical power they are 



1 " Sitzungsberichte der K. K. Akad. zu Wien," 1851, Bd. vi. p. 554. 



2 It is in principle impossible to eliminate chromatic aberration by a com- 

 bination of two or more plano-convex lenses ; by judicious selection of the 

 relative distances the parallelism of the red and violet pencils, or of their 

 axes, is at most effected, as we have already explained when describing the 

 eye-pieces. An intersection of the optically effective pencils of rays between 

 the refracting surfaces does not take place. In this respect the explanation 

 given by Harting (" Mikroskop," p. 113, and Fig. 8 of the annexed table) is 

 entirely erroneous, because the path of the rays assumed by him for the 

 observing eye is impossible. How can the emerging pencils of light reach the 

 retina through a pupil of 2 to 3 mm. in diameter, if they diverge as in the 

 figure referred to ? 



