Mr. W. R. Grove o?? Aplanatic Teiescopes. 163 



small ones, 2 inches and under. Viscous Canada balsam and 

 castor-oil mixed and heated and cooled repeatedly, become ulti- 

 mately solid. 



I have nothing better to propose as a substance actually ex- 

 perimented on than the above two cements, viz. rosin and cas- 

 tor-oil, and fused Canada balsam and castor-oil. Oil of cassia I 

 abandoned after an experiment or two, on account of its lim- 

 pidity : doubtless this and other substances might be used in 

 combination with a fused resinous substance; but I have not 

 tried it. Sulphuret of carbon, the third liquid in Brewster's 

 list, mentioned by Mr, "VTray, seems to me wholly impracticable 

 from its great volatility. 



Although the substances at the head of Brewster's list, speak- 

 ing generally, have a greater dispersive action on the more re- 

 frangible rays of the spectrum, and those at the bottom of it on 

 the less refrangible rays (see Brewster's diagram of oil-of-cassia 

 and sulphuric-acid spectra, "Optics," p. 78), yet each substance 

 has peculiarities and must be experimentally tried by the old 

 plan of two prisms, or similar methods, before it is used. None 

 in theory absolutely correct the irrationality ; but several do so 

 practically, and the problem is, for each specimen of flint and 

 crown glass used, to select a substance which sufficiently cor- 

 rects the secondary spectrum and can be used by heat-fusion so 

 as to form a tough solid when cold. To this point, in my opinion 

 the most important, Mr. Wray does not allude. Of liquids, I 

 have found castor-oil the most durable ; and if the lenses be in 

 an iron well-fitting cell (for it acts on brass rapidly), I am not 

 sure that such material might not be used alone with advantage ; 

 but with large object-glasses it is excessively difficult to prevent 

 the meniscus of castor-oil from being wedge-shaped and so gi- 

 ving a tail to stars and planets. When the size of the glasses is 

 notable, it is extremely difficult to remedy this defect. I pro- 

 ceed to mention some modes of overcoming difficulties of ma- 

 nipulation which cost me much labour; I omit my failures. 



1st. To apply solid transparent cements which melt by heat 

 to object-glasses without danger of breaking or unannealing the 

 glass. — An iron plate, about one-quarter of an inch in thickness, 

 and at least an inch greater in diameter than the glass, is placed 

 on a ring of metal and well levelled ; on it place a single piece of 

 soft filtering-paper, and on this the object-glass; the flint glass 

 being next the paper. Have the cement melted in a porcelain 

 vessel placed in a sand-bath, the temperature being just high 

 enough to fully melt it but not to carbonize it, and keep it melted 

 as short a time as practicable ; then heat uniformly, by a spirit- 

 lamp underneath the iron plate, until the object-glass above is 

 so hot that the hand can just bear it; remove the crown glass, 



M2 



