of the large Refracting Telescope just completed. 43 



larger, their power decreased. One of the difficulties was, that 

 the glass used for the object-glass could not be obtained as 

 perfect as such large instruments requn-e. In fact, the English 

 flint-dass has undular lines which disperse the light irregularly 

 in its passage through it. These streaks being more nume- 

 rous in a laiger and thicker glass than in a small one (whi st, 

 if the effect % to be increased, they ought to be less so), the 

 power of the object-glass was diminished if the instruments 

 were particularly large. The English crown-glass too, as in 

 fact every other kind of glass hitherto used, has those undu- 

 lar streaks, which, although not always visib e to the naked 

 eve will yet ^ive a false direction to the rays by an irregular 

 refraction. The Bavarian flint- and crown-glass, however, is 

 free from these streaks, and equally compact throughout : the 

 difference between the flint- and crown-glass being chiefly m 

 the greater power of dispersing the colours, and the propor- 

 tion of this power being in the English flint-glass, compared 

 to the common glass, as 3 to 2, but in the Bavarian as 4 to 2 : 

 the latter is also preferable in this respect m the given pro- 



^"^ There were not till the present time any fixed theoretic 

 principles for the construction of achromatic object-glasses : 

 and opticians were obliged, within a certain line, to rely on 

 chance, which made them polish a greater number of glasses, 

 and select those in which the faults were most compensated. 

 As the probability of this chance is much less in large glasses 

 than in small ones, even those of the middle size would have 

 been seldom perfect; and even with the best flint-glass^ the 

 construction of large achromatic object-glasses would have 

 been impracticable. The more important <=^"«^«;h.ch ren- 

 dered this process necessary are as follows: The theory ot 

 achromatic object-glasses being as yet imperfect; the means 

 formerly applied for ascertaining the powers of refraction and 

 dispersion of colours in the different species of glass which 

 ou ' ht to rest on a firm basis, not being sufficiently estabhshed ; 

 anS on account of the methods hitherto used for grmding 

 and polishing the glasses not being calculated to follow the 

 theory with that degree of exactness, as they ought, if a pal- 

 pable indistinctness should be avoided. 



\ All those impediments, however, together with many others 

 have now been successfully removed, partly by inventions and 

 partly by discoveries, to which we were led m pursuing this 

 object. I shall, however, perhaps find another opportunity for 

 entering more largely upon this subject. 



The object-glass of the great retractor, of which I am now 

 speaLg,'has^l08 Paris lines aperture, and 160 inches focus. 



