THE HliSTORY OF THE TELESCOPE. 1 Of) 



which should keep the surface true within a liiuit far transcendiug the 

 raiii.'C of measureiiieuts. Huyghens is the first who seems to liave (h»iie 

 (his, by polishiiiii upon a paste which was formed to the lilass and then 

 (hied, and by using only the central i)ortion of a hirge lens. In Italy 

 Canspani develoi)ed a system which he most jealously guarded as a 

 secret until his death, consisting of i)olishing with a dry powder on 

 paper cemented to the grinding tools. This method still survives in 

 i'aris to the exclusion of almost all others, and it is probably the best 

 for work which does not demand the highest scientific precision, 



Newtou however was the first to introduce a method which has 

 since been developed to a state of surprising delicacy. Casting about 

 for a means which should be sufticiently ''tender," to use his own ex- 

 ]»ression, for i)olishing the soft speculum metal, he fixed upon i)itcli, 

 shaped to the mirror while warm, as a bed to hold the polishing powder. 

 But the enormcms value of this substance lies not so much in the com- 

 l)arative immunity which it gives from scratching, but in the fact that 

 under slowly changing forces it is a liquid, but under those of short 

 duration it behaves like a hard and brittle solid. Thus it is possible 

 to slowly alter the shape of a lens while polishiug, in any desired direc- 

 tion. It was only after the ]nactical recognition of this fact that really 

 excellent lenses were nuich more than a question of good fortune. The 

 l)erfecting of this method belongs without doubt to the English of the 

 last century and the early i)art of this. In the Philosoi)hical Transac- 

 tions, we find many long papers relating to this art, contributed by 

 skillful and suc<-essful anuiteurs. We may therefore regard the tech- 

 i!i(pu' of the art of lens-making as practically complete at the middle of 

 this century and as common property, so that success no longer de- 

 pends upon the holding of some special or secret method. 



We are now (after this, I fear, somewhat dry discussion of a necessary 

 ])oint) in a condition to explain the differences between the processes 

 pursued by most telescope niakersand that of the maker of the Carletou 

 ("olh'ge telescoi)e. 



This is the ordinary method: After securing perfect pieces of glass, 

 crown and flint, as like as i)()ssible to those generally used, and having 

 fixed upon the general shape of the lenses, a guess is made as to the 

 lin>])er radii of the foui- surfaces to detennme the desired focal length 

 and corrections both for color and spherical aberration. The success 

 of this guess has much to <lo with the necessary outlay of labor, and 

 fherei'ore ])ast expi'rience is of great value as a guide. iMter working 

 tlic lour surfaces to the dimensions pro\ isionally adopted so far as to 

 admit of lairly good seeing through the objective, an examination of the 

 errors is made. Should the errors of color be S(> small that their final 

 correction w ill not make the telesco])e more than from 3 to 10 per cent 

 greater or less than the desired focal length, the crown lens will i)rob 

 ably be comi)leted in accordance with the prctvisional figures. I'hen 

 the Hint lens will be modified in such a tlirei^tion as will tend to correct 



