A SILVEF.ED GLASS TELESCOPE. 



27 



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

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

 as good pictures. A photograph which is magnified to 3 feet, represents a power 

 of 380. As the original negative taken at the focus of the mirror is not quite 1| 

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

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



The light collecting power of an unsilvered mirror is quite surprising. With a 

 15g inch, the companion of a Lyrse can be perceived, though it is only of the 

 eleventh magnitude. The moon and other bright 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 description, 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 like a cross, and this though the curva- 

 ture be so slight as hardly to be perceptible by ordinary means. For a long time 

 I used a piece 3x5 inches, which was cut from the centre of a large looking-glass 

 accidentally broken, but eventually found that by grinding three 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 were tested by being put in the 

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

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

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



§2. THE TELESCOPE MOUNTING. 



The telescope is mounted as an altitude and azimuth instrument, but in a manner 

 that causes it to differ from the usual instrument of that kind. The essential 

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

 stationary at all altitudes, the observer always looking 

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

 convenient and constrained positions. 



The stationary eye-piece mounting was first used by 

 Miss Caroline Herschel, who had a 27 inch Newtonian 

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



Subsequently it was applied to a large telescope by Mr. 

 Nasmyth, the eminent engineer, but no details of his con- 

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

 mgs of the moon, which are said to be excellently executed. 



When it became necessary to determine how my tele- 

 scope should be mounted, I was strongly urged to make it miss Hersohei's Telescope. 



^ A right-angled prism cannot be used with advantage to replace the plane silvered 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. 



