imtkjlui^ object-glasses. 337 



riod ; and it has likewise been supposed that the fluid itself 

 might be liable to some change which might impair the per- 

 formance of the telescope. 



Amply sufficient experience has been had, however, to show 

 that telescopes may be constructed on this principle so as to 

 be liable to neither of these objections. A telescope of this 

 kind is now in my possession, in the object-glass of which the 

 fluid has been enclosed for upwards of thirty years, without suf- 

 fering any diminution of quantity, and its performance continues 

 decidedly superior to that of any common achromatic telescope 

 of the same focal length. In this instrument, however, a slight 

 change did take place in the fluid within a few years after it 

 was constructed, owing to a minute quantity of one of the 

 substances contained in it being deposited in a crystalline form. 

 By this the refractive and dispersive powers of the fluid ne- 

 cessarily underwent a slight alteration, and the correction of 

 ' the aberrations is certainly not so perfect as at first ; the de- 

 gree of imperfection, however, is barely appreciable, and the 

 correction of the colour is still considerably more perfect than 

 is possible in the common achromatic telescope. 



In order to obviate entirely this last mentioned defect, ex- 

 periments were made for the purpose of discovering a sub- 

 stance capable of supplying the place of the one disposed to 

 crystallize, and free from that defect. In this complete suc- 

 cess has been attained, and object-glasses have remained with 

 this fluid enclosed in them for twenty-one years without suf- 

 fering any alteration, and which continued to perform as well 

 as at first. This fluid, which is the one now adopted, produ- 

 ces a perfect correction of the chromatic aberration, dispers- 

 ing the variously coloured rays of the spectrum in exactly the 

 same proportion as crown glass. Its refractive power is some- 

 what greater than that of the former, which rendered a differ- 

 ent set of curves necessary for the correction of the spherical 

 aberration ; and as the services of a glass-grinder could not at 

 that time be commanded, these last mentioned object-glasses 

 were not altogether perfect in this respect, on which account 

 they were taken to pieces for the purpose of being rendered 

 complete, one, however, being reserved for the purpose of 

 proving the durability of the construction. 



