14: THE MICROSCOPE AND ITS REVELATIONS. 



ing principle to the Microscope, resumed his original labors on hearing- 

 of the success of MM. Selligues and Chevalier; and, by working on their 

 plan, he produced, in 1827, an achromatic combination which surpassed 

 anything 6J. the same kind that had been previously executed. And 

 these were soon rivalled by the objectives produced in London by Andrew 

 Koss and Powell. 



16. It was in this country that the next important improvements 

 originated; these being the result of the theoretical investigations of Mr. 

 J. J. Lister, 1 which Ted him to the discovery of certain properties in 

 Achromatic combinations that had not been previously detected. Under 

 his guidance, Mr. James Smith, soon followed by other Opticians, 

 succeeded in producing combinations far superior to any which had been 

 previously executed, both in extent of aperture, flatness of field, and 

 completeness of correction; and continued progress has been since made 

 in the same direction by the like combination of theoretical acumen with 

 manipulative skill. 



17. The enlargement of the Angle of Aperture, and the greater com- 

 pleteness of the corrections, first obtained by the adoption of Mr. Lister's 

 principles, soon rendered sensible an imperfection in the performance of" 

 these lenses under certain circumstances, which had previously passed 

 unnoticed; and the important discovery was made by Mr. A. Koss that 

 a very obvious difference exists in the precision of the image, according 

 as the object is viewed, with or without a covering of talc or thin glass; 

 an Object-glass which is perfectly adapted to either of these conditions, 

 being sensibly defective under the other. The mode in which this differ- 

 ence arises is explained by Mr. Koss 2 as follows: Let o (Fig. 11) be any 

 point of an object; o p the axial ray of the pencil that diverges from itr 

 and o T, o T', two diverging rays, the one near to, the other remote from, 

 the axial ray. Now if G G G Q represent the section of a piece of thin 

 glass intervening between the object and the object-glass, the rays o T 

 and o T' will be refracted in their passage through it, in the directions 

 T B, T' E'; and on emerging from it again, they will pass on towards E 

 and E'. Now if the course of these emergent rays be traced backwards, 

 as by the dotted lines, the ray E R will 'seem to have issued from x, and 

 the ray E' R' from Y; and the difference x Y, which is called ' negative 

 aberration,' is quite sufficient to disturb the previous balance of the 

 aberrations of the composite lens of the object-glass. The requisite cor- 

 rection may be effected, as Mr. Ross pointed out, by giving to the front 

 pair (Fig. 10, 1) of the three of which the Objective is composed, an 

 excess of 'positive aberration' (i. e., by under-corfecting it, and by 

 giving to the other two pairs (2, 3) an excess of * negative aberration' 

 (i. e., by over-correcting them), and by making the distance between the 

 former and the latter susceptible of alteration by means of a screw collar 

 ( 140). For when the front pair is approximated most nearly to the 

 other two, and its distance from the object is increased, its positive 

 aberration is more strongly exerted upon the other pairs than it is when 

 the distance between the lenses is increased, and the distance between 

 the front pair and the object is diminished. Consequently, if the lenses 

 have been so adjusted that their correction is perfect for an uncovered 

 object, the approximation of the front lens to the others will give to the 

 whole combination an excess of positive aberration, which will neutralize 



'See his Memoir in the "Philosophical Transactions" for 1829. 

 2 " Transactions of the Society of Arts," vol. li. 



