340 Mr Blair on Achromatic Telescopes. 



with the most perfect distinctness a power of two hundred 

 times, and are most effective in reading a printed paper dur- 

 ing bright sunshine with a power of three hundred times. Not 

 having any convenience for observing, I have not yet been able 

 to make trial of their performance in separating double stars. 

 I am inclined to believe, however, that the terrestrial objects 

 which I am in the habit of using are even more rigid tests of 

 the performance of the telescope than the fixed stars. The 

 object which I think best for proving both the figures of the 

 lenses and the accurate removal of the aberrations, is a minute 

 image of the sun reflected from a small convex surface, such as 

 a thermometer bulb blown of black enamel ; this, when viewed 

 at a proper distance, has exactly the appearance of a fixed star, 

 excepting that it has the advantage of being capable of being 

 made much brighter. This object, when of sufficient bright- 

 ness, is by much the most delicate test of the correction of the 

 colour that can be used ; and as the telescopes I have con- 

 structed show such an object extremely small and perfectly 

 round, I know from experience that they may safely be relied 

 on for separating the most difficult double stars. The aper- 

 tures of these instruments might have been made still consi- 

 derably larger than they are, had I not been prevented by 

 the want of glass of a sufficient thickness. As it is possible, 

 however, that a minute quantity of the spherical aberration 

 which remains in those parts of the lens which are about half 

 way between the circumference and the axis, and which it is 

 impossible altogether to get rid of, may in extreme cases be- 

 come sensible, it will on that account not be advisable to push 

 the increase of aperture to an immoderate extent. In the largest 

 apertures which have been tried, however, I have not been able 

 to discover any traces of the green and purple which constitute 

 the secondary spectrum. 



I am at present engaged in the construction of a telescope 

 five and three-eight inches in aperture, and five feet in focal 

 length ; but as I have not yet the advantage of employing a 

 machine for the purpose of giving to the lenses the very accu- 

 rate figure which an instrument of that size demands, very 

 great success is perhaps not to be expected. I should wish 

 likewise to make the aperture for that focal, length to exceed 



