Dr. Ure on the Spectd^ of Telescopes. 53 



drawing, by the constructors of the instrument, will explain 



more fully the above-mentioned mechanism. 



I am, my dear Sir, yours truly, 



Andrew Ure. 



Sir, Glasgow, 4th March, 1823. 



According to your request, we send you a description of the 

 mode of mounting the large speculum of our telescope. 



In one of the Gregorian construction of six inches aperture, 

 we mounted the speculum on the Herschelian plan, but found 

 from experience with it at the Glasgow Observatory, as well as 

 with those made by that admirable astronomer. Sir William 

 Herschel, that this mode was liable to many objections, being 

 apt to suffer from dust falling from one's clothes, or drops of 

 water from the cover in a dewy night ; and its being easily 

 touched by the finger of those, who were not aware of the mis- 

 chief which may result to the delicate polish of a speculum 

 from a moist hand. For the information of such persons as 

 have not examined Sir William Herschel's telescopes, it may 

 be necessary to state, that a portion of the three upper staves 

 of the octagon-tube is cut through above the speculum, and 

 hinged in one piece to form a moveable door, of sufficient size 

 to admit of the speculum cover being readily applied or re- 

 moved. 



To get rid of the above-mentioned inconveniences, we fitted 

 the speculum into a brass ring, furnished with a channel in 

 front to receive the edge of the cover; the speculum itself 

 being introduced from behind, and its back fixed in the usual 

 manner. The lid or cover is formed of three pieces of brass, 

 neatly fitted and hinged together. They are of such a size, 

 that when lying down on the sides of the tube, the central 

 segment of the three applies accurately to the inferior stave of 

 the octagon, and the other two pieces rest inclined on the two 

 staves to the right and left hand uf the bottom one. In this 

 position, it can intercept none of the light moving in the teles- 

 copic cylinder. 



In the same line with the centre of the hinge, a square rod 

 of iron is attached to the middle segment of the cover, projecting 



