360 OBJECTIVES, EYE-PIECES, THE APERTOMETEK 



it will be surrounded by a corna, and even the portion of the flame- 

 which is in focus will lack brightness. But with the convex side to- 

 wards the flame it will be found that in the image on the card the 

 coma is greatly reduced, and the image of the flame brightened. 

 The reason for this is, as already stated, that the spherical aberration 

 is four times as great when the convex side of the lens is towards 

 the card. 



The practice of these simple tests will be most instructive to 

 those unfamiliar with the optical principles on which an objective is 

 constructed. They make plain that an over -corrected lens is one 

 which brings its peripheral rays to a longer focus than its central, vide 

 fig. 24, p. 20. But a cover-glass produces over-correction, therefore 

 the means employed to neutralise the error is by the under-cor- 

 rection of the objective. If, however, the objective employed 

 should be unprovided with such means of correction, the eye-piece 

 must be brought nearer the objective, which will effect the same 

 result. 1 



Still confining our consideration to the year 1837, we find that a 

 further improvement was made by Lister, who employed a triple 

 front combination. This consisted of two crown piano-con vexes with 

 a flint plano-concave between them. The result of this was the 

 increase of the aperture of an inch-focus objective to 22. 



An illustration of the mode of construction of these lenses is 

 given in fig. 314, which is drawn from an early -inch objective by 

 Andrew Ross, having bayonet-catch correction adjustment. In 1842 

 a ^-inch of 44, a J-inch of 63, and a ^--inch of 74 were made 

 upon the same lines. The method for computing these fronts i& 

 given by Mr. Nelson in the 'Journ. R. M. S.,' 1898, p. 160 et seq. 



In 1841 the Royal Microscopical Society ordered a microscope 

 from each of the before-mentioned leading opticians. The objectives 

 supplied with these are still extant, representing with moral certainty 

 the very best work of the several makers ; they are consequently 

 valuable as reliable specimens of the best work of the period. 



The objectives supplied by James Smith have the peculiarity of 

 being separating lenses. 



The lowest power is about 1 J-inch focus. When this is used 

 alone a diaphragm is slid over the front to limit the aperture, but 

 we are unable to say what that limit was, since the diaphragm has 

 been lost. By placing another front where the diaphragm would 

 have been, the new combination becomes an ^-inch focus, while 

 yet another front may be substituted, making the objective a J-inch 

 focus. This latter front consists of two pairs, and it is provided 

 with a graduated screw-collar adjustment which separates these 

 pairs, but the arrangement is of a very primitive order. 



This object-glass will divide the podura marks in a milky field 

 with a full cone, and the field is much curved. 



There is also a separating IJ-inch and f-inch which is good 

 while the T \-inch and the J-inch may be considered fair. 



The lenses supplied by Andrew Ross are a good 2 -inch and a. 



1 Under-correction is also known as ' positive aberration ; ' over-correction as 

 negative aberration.' 



