432 EXPERIMENTAL INVESTIGATIONS. 



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 differ- 

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

 degree the chromatic without increasing the spherical 

 aberration.' 



Before the date of the above communication I had used 

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

 three-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 three inches aperture and forty inches 

 focus, the focus without the cement being forty-two inches ; 

 this telescope was a very good one : it divided y Leonis, 7 

 Virginis, and other stars of that class ; the secondary spec- 

 trum was extremely slight, and capable of being detected only 

 by a practised eye on a bright spot, such us 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 objection 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 necessarily 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 lique- 

 fied by heat and became solid when cold ; Canada balsam 

 in its ordinary state was therefore objectionable, but when 



