:>?2 ACCESSORY APPARATUS 



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powers. where there is plenty of room for the light to pass between 

 the objective ,-ui.l the object. The ingenious use of the bull's-eye 

 emploxed by .Mr. .lames Smith, as detailed above, increases the possi- 

 bility of magnilication. luit it needs practice and care. With the 

 great improvement which has been eifected in objectives and con- 

 densers the need of a bull's-eye which should give the minimum of 

 aberration has become a desideratum; and Mr. Nelson has calculated 

 ami had constructed a doublet bull's-eye which gives admirable 

 results, There are described in most treatises on optics doublets 

 devised by Herschel which are said to be of 'no aberration.' Mr. 

 Nelson has shown ( Journ. Q. M. S.,' vol. vi. ser. ii. p. 197, 1896) 

 that they are by no means free from spherical aberration, and that 

 their forms are such as will not even yield a minimum amount of 

 such aberration; also that there is a numerical error in the focal 

 lengt h of the high-power doublet. He has computed that the spheri- 



cal aberration in the Herschel doublets amounts to '296^-, and 



Ji 



he gives the following formula for a combination, the spherical 



11- 



aberrationof \vhichis "207- ; or 30 per cent, less than in either of 



r 



those proposed by Sir John Herschel. 



Boro-silicate glass, Jena catalogue No. 5 ; yu = l - 51 . 



=1=64-0. 

 fy 



1st lens crossed, /= + 2'359) ,. 



s =_ir,-o7Hl (llametei " 2 ' 1; 



-'ml lens meniscus , r= + 1'280| ,. 



., i0t f diameter I'o. 

 s= + 3 - 4o4 ) 



Distance between the lenses <>.">. eipiivalent focus 2 - 0, working 

 distance or back focus 1 ">.">, total aberration -1035, clear aperture 

 2'0, angle 62. The second (Jauss point of the combination is close 

 to the posterior .surface of t he crossed lens. 



As there are some microscopists who might require a combina- 

 tion of this kind, but with a different focal length, and who are 

 unable to transpose the formula, (he following rule may be of use. 

 Halve all the radii and diainet ITS and multiply the results by the local 

 length that is required. A>"//////r. - Required a doublet on this 

 formula with 3^ inches of equivalent focus. Halving the data for 

 the crossed lens in the given formula, we have = + ! 1795, 

 s= 7'539, diameter I ().">: multiplying these results by .'!.', \\e 

 olilnin y=+-H2K, ,s-= 2t)-3S(i. diameter 3'7. Treat the meniscus 

 in thi same wav : the lens distance ma v with advantage be kept 



05. 



The following hull's eye is not so expensive to manufacture, and 

 mavoiilliat accounl i>e preferred to the doublet of minimum aber- 

 ration just described. Its form, though of minimum aberration for 

 t\\o piano convex lenses, possesses 13 per cent, more aberration than 



the fiil-lner. It \\illon fchlS aCCOUnI li"t lie possihlc to olltaill SUcll 



an even and unlimken disc of light with this form of bull's eye as 

 with the Other. The data are as follows. 



