TELESCOPES AT HONOLULU. 39 



(4.) A telescope of 3 inches aperture and 45 (?) inches focal length, by 

 T. COOKE, on a tripod stand, my own property. 



These four telescopes, in common with most of the telescopes employed on 

 the Transit of Venus Expeditions, were fitted by Messrs. Troughton and 

 Simms with special means for observing the Sun, which, although well known, 

 it is proper to describe here. 



The cone of rays from the object glass is intercepted, before the primary 

 focus, by a glass prism, one face of which is adjusted to an inclination of 45 

 to the optic axis of the telescope, so as to reflect a portion of the light 

 approximately at right angles to the optic axis. About 90 per cent, of the 

 light and heat passes through the prism and is dispersed. The eye-pieces 

 of whatever character are inserted immediately above the prism. The 6-inch 

 and the 4^-inch equatorials were both fitted with new double-image micro- 

 meters by Messrs. Troughton and Simms. Plate III. represents one of these 

 instruments in position. The light is further toned down by an achromatised 

 wedge of neutral tint glass, placed between the two lenses nearest the eye. 

 The equality of the intensity of the two images is adjusted by the screw 

 which has the power of slightly altering the position of the axis of the 

 micrometer with regard to the optic axis of the object glass. The adjust- 

 ment of the reflecting prism is perfect when the micrometer can be rotated 

 about its own axis without altering the relative intensity of the two images. 

 This adjustment is liable to derangement from the circumstances that the 

 prism must not be too closely confined in its cell, as it is liable to become 

 very much heated when in use. No difficulty was experienced with either 

 telescope at Honolulu in regulating the intensity of the two images. This 

 double-image micrometer is described at length in the Memoirs of the Royal 

 Astronomical Society, XV. 199, and in the Monthly Notices, VI. 229, and 

 X. 160. One half of the divided lens was fixed, the other was moved by the 

 micrometer screw. They were not, however, provided with position-circles. 



Both the equatorials were mounted upon iron pillars, which stood upon 

 piers of brickwork built up from the coral rock. The driving clocks, which 

 performed well, were adjusted to the Sun's apparent motion. The 6-inch 

 instrument was sheltered by a wooden hut, the roof of which could be 

 removed in pieces. The 4i-inch instrument was protected, when not in use, 

 by a small hut of wood and canvas, which was removed entirely when the 

 instrument was in use. 



The Secondary Clock Dent 2012, the pendulum rod of which was of 

 wood, was mounted in the 6-inch equatorial hut, and was compared, when 



