162 Mr. W. R. Grove onAplanatic Telescopes. 



Cooke, the well-known optician. Such publication may assist 

 others in promoting this important object. , Though at work 

 on the subject for some time previously, the first published 

 record on the subject I can lay my hand on is in the Monthly 

 Astronomical Notices for January 14, 1853, in which occurs 

 the following paragraph : — "In some cases where the inner 

 curves of the flint- and crown-glasses approximate, Mr. Grove 

 had employed with success a highly refracting cement made of 

 very clear rosin and castor-oil, which, acting as a third lens or 

 convex meniscus of a medium dispersing the coloured spaces 

 differently from the other two lenses, corrected to a very great 

 degree the chromatic without increasing the spherical aberra- 

 tion." 



Before the date of the above communication, I had used 

 for a similar purpose copal varnish between the lenses ; and a 

 3-inch object-glass, of four-feet focus, so corrected by me was 

 used for several years by Mr. B. Hill of Swansea. Scarcely any 

 secondary colour was perceptible ; but in time, beginning at the 

 second year, small bubbles appeared between the lenses, arising 

 from the drying of the varnish, though the meniscus of it was 

 so thin that it only shortened the focus 07 of an inch. It was 

 this defect that led me to use the rosin and castor-oil, which 

 formed a tough cement practically unalterable. The telescope 

 alluded to in the above extract from the Monthly Notices was of 

 3 inches aperture and 40 inches focus, the focus without the 

 cement being 42 inches; this telescope was a very good one: it ; 

 divided 7Leonis, y Virginia, and other stars of that class; the 

 secondary spectrum was extremely slight, and capable of being 

 detected only by a practised eye on a bright spot, such as the 

 planet Venus. I used it for some time ; but wishing to make it 

 more perfect I broke it, as I have done many others. I was 

 convinced from the above and other results that oils or liquids 

 which dry or shrink more or less in time, were open to objec- 

 tion similarly to Blair's object-glasses, where the correction is by 

 a concave lens of liquid instead of a convex meniscus, and the 

 substances for correction used with glass are therefore neces- 

 sarily chosen from towards the bottom instead of the top of 

 Brewster's list (" Optics/' Lardner's Cycl. pp. 374 and 375), 

 in which oil of cassia stands first and sulphuric acid last. I 

 was satisfied that the substance to be aimed at should be one 

 which liquefied by heat and became solid when cold ; Canada 

 balsam in its ordinary state was therefore objectionable, but 

 when obtained hard and then tempered by admixture with castor- 

 oil, so as to form a substance just taking, when cold, the im- 

 pression of the nail, it succeeded very well, though from my ex- 

 perience of the accidents with large glasses I only tried it on 



