28 Lord Oxmantown on a new Reflecting Telescope. 



ground and polished in the common way, till by repeated 

 trials it was found to be of a good spherical figure. The 

 small screws i i i, were then drawn back, the speculum placed 

 in the tube, and the spindles turned round a certain number 

 of times, by which the centre part of the speculum was made 

 to approach nearer the plate AB, by a quantity ascertained in 

 a manner I shall presently point out. The two images were 

 then made to coincide, and the image was then found to be 

 apparently as distinct as either image had been when separate. 

 It is necessary to observe, that, in order to effect the adjustment 

 properly, each image must be brought to the same degree of 

 brightness, which can be accomplished by shades on the mouth 

 of the telescope, and that no higher power should be used than 

 each metal, when separately employed, can bear with distinctness. 

 I rather think that instruments of this construction will pretty 

 frequently require adjustment; however, this is easily effected 

 in the space of two or three minutes. The first I attempted 

 consisted of a solid metal surrounded by two rings. Owing to 

 a defect in the mechanism it required very frequent adjust- 

 ments, the smallest shock displacing the images. The one I 

 have described is almost entirely free from this defect, remain- 

 ing in perfect adjustment even after very violent shocks. I 

 have a speculum like the first I made, consisting of three parts, 

 which is almost ready for grinding ; and I expect it will turn 

 out well. I have not yet perceived any ill effects from expan- 

 sion and contraction, which was the difficulty I most appre- 

 hended. Whether they will become perceptible in instruments 

 of higher powers, or whether, if perceived, means may or may 

 not be devised of obviating them, can only be ascertained by 

 future trials. On my return from Parliament, if other avoca- 

 tions do not interfere, I propose to construct a speculum in 

 three parts of eighteen inches aperture and twelve feet focal 

 length ; — this will be giving the experiment a fair trial on a 

 large scale. It may perhaps be as well to observe, that I do 

 not think the principle of subdividing the aberration can be 

 applied with advantage to small instruments. The object to 

 be gained by it is a diminution of the focal length with a given 

 aperture and power, and this is by no means desirable in small 

 instruments, as it forces us to make use of deep eye-glasses, 

 which are on many accounts objectionable. 



