NEW CLASS OF A PLAN ATI C TELESCOPES. 433 



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 

 experience of the accidents with large glasses, I only tried it 

 on small ones, two inches and under. Viscous Canada balsam 

 and castor-oil, mixed and heated and cooled repeatedly, be- 

 came ultimately solid. 



I have nothing better to propose as a substance actually 

 experimented on than the above two cements, viz. rosin and 

 castor-oil, and fused Canada balsam and castor-oil. Oil of 

 cassia I abandoned after an experiment or two, on account of 

 its limpidity. 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. Wray, seems to me wholly 

 impracticable, from its great volatility. 



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

 speaking generally, have a greater dispersive action on the 

 more refrangible rays of the spectrum, and those at the bot- 

 tom 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 corrects 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 giving a tail to stars and planets. 

 When the size of the glasses is notable it is extremely difficult 

 to remedy this defect. I proceed to mention some modes of 

 overcoming difficulties of manipulation which cost me much 

 labour ; I omit my failures. 



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