S96 Mr Blair on Achromatic Telescopes 



^ Tlie method employed for enclosing the fluid has proved 

 so efFectiial, that not a single instance has occurred of its 

 escape from a finished object-glass ; and there were ten or 

 twelve small ones of about two inches aperture, which remain- 

 ed filled with the first mentioned fluid for upwards of twenty- 

 six years without its suffering any diminution. These were 

 afterwards taken down for the purpose of being filled with 

 the improved fluid, at the time when I was receiving instruc- 

 tions from my father in the method ol putting this invention 

 into practice. 



When object-glasses of this kind are of considerable diame^ 

 ter, it is difficult to prevent the intrusion of a small quantity 

 of the cement, used for the enclosing, into the space containing 

 the fluid. This takes place, however, only to a very small ex- 

 tent, presenting the appearance of a few minute globules only 

 adhering to the edges of the object-glass, which, if objected to, 

 may be kept out of sight by slightly reducing the aperture. 

 In some cases a small bubble of air is present ; this, however, 

 does not arise from any escape of the fluid, but is present from 

 the first, and when so present, the intrusion of the cement does 

 not take place. When the object-glasses do not much exceed 

 two inches in aperture, they may generally be made without 

 either bubble of air or intrusion of cement. 



On the whole, if there be any difference in regard to per- 

 manency, it would seem to be rather in favour of an object- 

 glass composed of two lenses with an enclosed fluid, as it will 

 be impervious to the dust which gets between the lenses of 

 common achromatics. * 



The superiority of these instruments, when accurately con- 

 structed, over common achromatic telescopes, is greater than 

 might be at first expected, even from a perfect removal of the 

 colour. It is to be kept in mind, however, that the colours of 

 a secondary spectrum, produced by a combination of crown 

 and flint-glass, are more injurious to the performance of a tele- 



* When the object-glasses with fluids are properly constructed, so as to 

 correct the spherical aberrationj three lenses are required, and the fluid 

 is only between two of them, so tliat in that case they will be liable to the 

 intrusion of dust between two of the glasses in the same way as common 

 achromatics. 



