182 The Dialyte Telescope. 



not only founded upon a self-evident principle, but (unknown 

 of course to Santini) was actually found to answer perfectly in 

 practice. In addition to these conveniences it possesses' 

 another of considerable importance. We have supposed the 

 corrector to exert merely a concave dispersion, without general 

 refractive power; but this is by no means necessary. Its 

 curves may be so proportioned as to give it a concave refrac- 

 tion also ; the result of which will be the enlargement of the 

 focal image, or, which is the same thing in practice, the short- 

 ening of the instrument materially, with the same aperture 

 and magnifying power. 



Possessing advantages so much more considerable, on the 

 whole, than its defects, it seems remarkable that this con- 

 struction should have attracted so little notice. It was carried 

 into effect by Dollond, for Rogers, on a scale of five inches, 

 with success ; but I have not heard of another instance of its 

 being even attempted in this country till of late years one was 

 worked by an excellent optician, Wray, for Mr. Buckingham, 

 the constructor of the magnificent achromatic of 20 inches aper- 

 ture, and about 30 feet focus, which made so conspicuous an 

 appearance in the nave of the International Exhibition. It 

 had 8^ inches of clear aperture, and its possessor has spoken 

 very highly of its defining quality. But with these exceptions, 

 it seems to have been ignored in England. On the Continent, 

 however, its advantages were more fully appreciated. At the 

 recommendation of the celebrated German mathematician, Yon 

 Littrow, it was taken up towards 1833* by an optician of 

 Vienna, named Plossl, by whom a great number have been 

 constructed, and some of considerable magnitude. About the 

 year 1850 one was completed of 6*4 inches for Jassy, another 

 for Biczke of 8 - 5 inches, and another for Athens of 8 inches. 

 The latter has now been for some years under the charge of 

 the celebrated Dr. Schmidt, who does not, however, speak 

 very favourably of its present condition. One of 6*4 inches 

 was also ordered for the magnificent establishment at Poul- 

 kowa. The Baron Dembowski, well known as a very indus- 

 trious and careful measurer of double stars, employed one for 

 a long time at Naples of 5^- inches aperture, and 5 feet 4 inches 

 focus, which would perfectly separate £ Bootis, and elongate 

 7 2 Andromeda?. His subsequent adoption of an ordinary 

 Merz achromatic of larger size (see Intellectual Observer, 

 vi. 59), may however be taken as a tacit preference of the 

 other construction. But the chef-d'oeuvre of Plossl appears to 

 have been an instrument made for the Sultan about 1851, 

 which had an aperture of fully 1 1 inches, and focal length of 



* Herschel's date is 1839, when perhaps it became generally known, but that 

 in the text is from a contemporary publication by Gruithuisen. 



