3 to Mr. Nixon on the Measurement of the 



cylinder, as seen through the telescope, appeai'ed to be bi- 

 sected by the intersecting point of the threads of the latter. 

 This being effected, the stand was glued at the sides (only) to 

 the surface of the plank, and the slight deviation in the bi- 

 section of the wire, occasioned by the setting of the glue, recti- 

 fied by the lateral adjusting screws (I I) of the telescope. 



Illumination of the Wires. — In the daytime a square foot of 

 white pasteboard set up on the plank at a distance of about 

 six inches from the northern object-glass of the cylinder, was 

 so inclined as to reflect the light from the window up the cy- 

 linder and telescope. At night, or on a gloomy day, the 

 lantern (M) of a transit instrument was substituted, and so 

 situated that the tube (N) containing the lens was of the 

 height and in a line with the cylinder and telescope. In lieu 

 of a lamp, a white wax taper, about one-third of an inch thick 

 and five inches long, was fixed within the socket of the lantern, 

 the summit of the taper standing an inch or two above the 

 level of the lens-tube (N). The wires of the cylinder and 

 threads of the telescope could now be seen, on looking through 

 the eye-glass of the latter, with the utmost distinctness in 

 front of the pale taper mildly, illuminated by the light reflected 

 from the tinned sides of the lantern. When the taper had 

 burnt down nearly to the level of the lens-tube, the light be- 

 came so intense that the wires appeared to split; and, on 

 the other hand, when the taper stood too high, they looked 

 dim and ill defined. In general, whether the lantern or paste- 

 board was used, a piece of black cloth was thrown loosely over 

 the object-glass of the telescope and the adjacent one of the 

 cylinder, its loose folds being so arranged as to exclude all 

 light, except that which passed through the further object- 

 glass of the cylinder. 



Adjustmejit of the Wires to the Sidereal Focus. — The tele- 

 scope, previously to its .being glued to the plank, was pointed, 

 when the ground was covered with snow, to some park rail- 

 ings at a distance of several miles, and the eye-tube pushed 

 in or drawn out until the railings could be seen with the 

 greatest distinctness*. The telescope being then considered 

 as properly adjusted, the spider threads were moved by the 

 handle (G) to such a distance from the eye-glass as produced 

 the clearest vision of the minute particles of dust adhering to 

 their sides. To confirm the accuracy of the latter adjustment, 

 the lantern was placed a little beyond the eye-glass, and the 

 threads viewed through the object-glass by an excellent 20- 

 inch {military) achromatic by DoUond, of which the sidei'eal 



• The telescope could not conveniently be adjusted upon a star. 



focus 



