180 The Dialyte Telescope. 



John HerscheFs tutor, Mr. Rogers.* In this construction the 

 object-glass is a single plate lens,, and the correction of the 

 two aberrations is produced by a combination of a plate and a 

 flint lens interposed in the cone of rays before they reach 

 the focus. To comprehend the method of doing this, we must 

 consider what would be the effect of placing together two 

 lenses, one convex and one concave, of equal focal lengths. 

 Then, if each were of the same kind of glass, or of glass 

 having the same amount : of dispersive action, the rays would 

 pass through quite colourless ; but if their dispersive powers 

 were unequal, though the general mass of rays would still be 

 transmitted parallel, yet colour would be produced in propor- 

 tion to the inequality of the dispersions, and in the direction 

 of the stronger of them. In the case, therefore, of a plate con- 

 vex and flint concave of equal foci, there would be colour due to 

 the superior dispersion of the flint, and acting in a concave 

 direction, or opposite to that of the object-lens. Now, supposing 

 this uncorrected colour to be equal in amount to that of the 

 object-lens, it is plain that if the two lenses producing it were 

 placed immediately behind the object-lens, they would destroy 

 its chromatic aberration, and we should have a combination 

 similar to Dollond's triple object-glass already described (In- 

 tellectual Observer, vi. 451). This, of course, would require 

 the flint disc to be of the full size. If, however, we could in- 

 crease the amount of the dispersion produced by the two com- 

 bined lenses, we might remove them from the object-lens 

 towards its focus, to a point where its convex or positive dis- 

 persion, increasing as it departs from its source, would be 

 exactly balanced by their negative excess of colour, and since 

 the cone of rays would there be smaller, we should gain in 

 respect of the size of the flint lens exactly in proportion as we 

 could remove it from the object-lens. But as the excess of 

 concave dispersion in the two combined lenses, which we may 

 collectively term the corrector, depends entirely upon their 

 focal length, we have only to shorten their foci, in order to in- 

 crease the excess of dispersion to any required amount ; and thus 

 in theory an object-lens of plate glass, however large, may have 

 its colour corrected by a disc of flint glass, however small. In 

 practice, however, a limit would soon be reached, as the 

 deepened curves required to produce the shortened foci would 

 introduce too much spherical aberration to be even approximately 

 corrected | for it is only with very small segments of a sphere, as 



* A similar idea had, it seems, occurred several years before to Sir David 

 Brewster, who had described and constructed lenses of this kind, and perceived 

 their applicability to telescopes ; but he admits that the invention was original 

 on the part of Mr. Rogers. The late Dr. Dick stated that a four-inch achromatic 

 was actually constructed on this principle with success by Mr. Wilson, of Glas- 

 gow, before he was aware that Mr. Eogers had proposed a similar plan. 



