A SILVERED GLASS TELESCOPE. 



27 



photographs taken with them. Although these are not as sharp as the image seen 

 in the telescope, yet it nuist not be supposed that an imperfect mirror will give just 

 as good pictures. A pliotogruph wliich is magnific>d to -i feet, represents a power 

 of 380. As the original negative taken at tlie focus of the mirror is not (|uite 1| 

 inch in diameter when the moon is at its mean distance, it has to be enlarged 

 about 25 times, and lias therefore to be very sharp to bear it. 



The liglit collecting power of an luisilvered mirror is quite surprising. AMth a 

 151 inch, the companion of a, Lyrae can be perceived, though it is only of the 

 eleventh magnitude. The moon and other briglit objects are seen with a purity 

 highly pleasing to the eye, some parts being even more visible than after silvering. 



In order to finish this tlescription, one part more of the optical apparatus requires 

 to be noticed — the plane mirrors. In the Newtonian reflector the image is rejected 

 out at the side of the tube by a flat surface placed at 45° with the optical axis of 

 the large concave.' If this secondary mirror is either convex or concave, it modifies 

 the image injuriously, causing a star to look lik(^ a cross, and tliis though the curva- 

 ture be so slight as hardly to be perceptil)le by ordinary means. For a long time 

 I used a piece 3 X 5 inches, which was cut from the centre of a large looking-glass 

 accidentally broken, l)ut eventually found that by grinding tliree pieces of 6 inches 

 in diameter against one another, and polishing them on very hard pitch, a nearer 

 approach to a true plane could be made. They Avere tested by l)eing put in the 

 telescope, and observing whether the focus was lengthened or sliortened, and also 

 by trial on a star. Wlien sufficiently good to bear these tests, a piece of the right 

 size was cut out witli a diamond, from the central parts. 



§ 2. THE TELESCOPE MOUNTING. 



The telescope is mounted as an altitude and azimuth instrinncnit 

 that causes it to differ from the usual instrument of that kind 

 feature is, that ////' eye-'piece or place of the sensitive plate is 

 statioaarij at all altltiuJes, the observer always looking 

 straight forward, and never having to stoop or assume in- 

 convenient and constrained positions. 



The stationary eye-piece moimting was first used by 

 Miss Caroline Herschel, who had a 27 inch Newtonian 

 arranged on that plan. Fig. 27. (Smyth's Celestial C'yck'.) 



Subsequently it was applied to a large telescope by Mr. 

 Nasmyth, tlu' emincmt (niginecr, but no details of his con- 

 struction have reached me. He used it for making draw- 

 ings of the moon, which are said to be excellcntlv executed. 



When it became necessary to determine how my tv\v- 

 scope should be mounted, I was strongly urged to make it 



1 )nt in a manner 

 The essential 



Miss Hersoliel's Telescope. 



* A right-angled prism cannot be used with advantage to rophice the plane silwercd mirrors, because 

 it transmits less light than they reflect, is more liable to injure the image, and the glass is apt to be 

 more or less colored. Its great size and cost, one three inches square on two faces being required 

 for my purposes, has also to be considered. 



