The Dialyte Telescope. 181 



we have already observed, tliat this kind of compensation can 

 be rendered perfect ; and hence a moderate ratio between the 

 apertures of the object-lens and corrector is preferable; if we 

 assumed one-half, which would give very practicable curves, 

 We could still employ a flint disc of only four inches (or a little 

 more, in order not merely to take in one point, but some 

 extent of field) to correct an object-lens of eight inches 

 aperture. 



This construction, as compared with the ordinary double 

 achromatic, is attended with several advantages and disadvan- 

 tages. The latter are obvious : the loss of light, perhaps 

 12 per cent., from two extra reflections and one transmis- 

 sion, and the greater chances of error in centring and working 

 two additional surfaces ; and, besides these, which are com- 

 mon to it with the triple achromatic, there is less possibility of 

 correcting pencils oblique to the axis, and much difficulty in 

 preventing the corrector from getting out of that perfect ad- 

 justment with the object-lens which is absolutely necessary. 

 Flexure, too, in the tube would, from the position of the cor- 

 rector, be more prejudicial; and this has been found an 

 annoyance even in the ordinary achromatic when the tube is 

 of great dimensions ; it was noticed by Airy in the magnifi- 

 cent telescope of Poulkowa, and no wonder, since in an expe- 

 riment by the German optician, E-eichenbachy a 24-pounder 

 gun was found to exhibit very sensible flexure when sus- 

 pended by its centre. But to these defects are opposed great 

 and peculiar advantages; not merely the diminished size of 

 the flint disc, which, in the present totally changed state of the 

 manufacture, is of much less importance, but an experimental 

 mode of final correction, which the constructors of the ordi- 

 nary achromatic would often be happy to have in their power. 

 For while it is difficult in the usual form to ascertain the pre- 

 cise ratio of dispersion in the plate and flint, and a slight mis- 

 take may require the reworking of one of the lenses, in the 

 dialytic construction it is only necessary to obtain a near ap- 

 proximation as to dispersion, the final correction being, as will 

 be readily seen, more easily and accurately accomplished by 

 varying the distance of the corrector from the object-lens, till 

 the most perfect achromaticity is obtained ; while, as regards 

 the spherical aberration, this, when such curves have been 

 given to the correcting lenses as may be expected to neutralize 

 it, may in like manner be finally, destroyed experimentally by 

 separating the lenses to a short distance from each other. 

 Such at least was the opinion of the inventor, Eogers. The 

 possibility of this latter ultimate adjustment has, indeed, been 

 questioned on mathematical grounds, by an eminent writer on 

 these subjects, Santini of Padua ; but Kogers's conclusion was 



