'44 Mr Dick's Description of a New lieflecting Telescope, 



maybe elevated or depressed, and firmly fixed in its proper posi- 

 tion ; and by tlie nut g^ it may be brought nearer to, or farther 

 from, the arm D. By the same apparatus, it is also rendered 

 capable of being moved either in a vertical or a horizontal direc- 

 tion: but when it is once adjusted to its proper position, it must 

 be firmly fixed, and requires no farther attention. The eye-piece 

 represented in this figure, is the one used for terrestrial objects, 

 which consists of the tubes belonging to a small achromatic tele- 

 scope. When an astronomical eye-piece is used, the length of 

 the instrument extends only to the point I. In looking through 

 this telescope, the right eye is applied at the point H, and the 

 observer's head is understood to be uncovered. For those who 

 use only the left eye, the arm would require to be placed on the 

 opposite side of the tube, or the tube, along with the arm, be 

 made to turn round 180 degrees. 



Fig. 2. Represents a front or rather an oblique view of the 

 instrument, in which the position of the speculum may be seen. 

 All the specula which I have fitted up in this form, having been 

 originally intended for Gregorian reflectors, have holes in their 

 centres. The eye-piece is, therefore, directed to a point nearly 

 equidistant from the hole to the exterior edge of the speculum, 

 that is, to the point a. In one of these instruments fitted up 

 with a four feet speculum, the line of vision is directed to the 

 point 5, on the opposite side of the speculum ; but, in this case, 

 the eye-tube is removed farther from the arm, than in the for- 

 mer case. The hole in the centre of the speculum is obviously 

 a defect in this construction of a reflecting telescope, as it pre- 

 vents us from obtaining the full advantage of the rays, which 

 fall near the centre of the mirror ; yet, the performance of the 

 instruments, even with this disadvantage, is superior to what we 

 should previously have been led to expect. 



The principal nicety in the construction of this instrument, 

 consists in the adjustment and proper direction of the eye-tube. 

 There is only one position, in which vision will be perfectly dis- 

 tinct. It must neither be too high nor too low, — it must be fix- 

 ed at a certain distance from the arm, — and must be directed 

 to a certain point of the speculum. This position must be ulti- 

 mately determined by experiment, when viewing terrestrial ob- 

 jects. A ^)erson unacquainted with this construction of the te- 



