Nov. 19, 1885] 



NATURE 



5 7 



were placed on improved chucks whose azimuthal and 

 vertical motions were effectually secured from dust and 

 injury, and left the shoulders of the pivots just sufficient 

 room for movint; without friction ; the Y's were morticed 

 upon 2 piers of Portland stone rising 5 feet 7 inches above 

 the floor, and which with their bases weighed a ton each. 

 The axis of the instrument was perforated at one end in 

 the usual way for the admission of light from a lamp at 

 night, but it also contained a contrivance for regulating, 

 by means of a milled head on the telescope tube, the light 

 falling on the wires ; and there was, moreover, a rack- 

 screen to the lamp for the same purpose. In the optical 

 focus were five principal vertical wires (besides two for the 

 Pole-star) crossed by one horizontal wire ; with a slide 

 and divided scale for bringing the axis of the eye tubes 

 exactly over the respective wires, and thereby destroying 



parallax. This part of the tube was also fitted with a 

 simple means for adjusting the eyepiece to the solar focus, 

 and for taking out the frame bearing the spider lines in 

 case they needed examination or repair. For setting the 

 telescope the eye-end was furnished with two circles, 5 

 inches in diameter, each provided with a level and show- 

 ing altitudes and zenith distances. But it is strongly 

 recommended that such circles should in all cases be 

 graduated and adjusted so as to show declinations.^ 



Setting circles attached to the eye-ends of telescopes 

 are so e.xtremely convenient for approximate settings that 

 it is a matter of surprise that they are not more generally 

 used. They are thought to have been invented by 

 Troughton, and to have been first applied in 1S16 to the 

 Greenwich transit instrument. As to this, Smyth has a 

 note as follows :— " Mr. Jones lent me a note-book of the 



late Mr. Walker, of Eidouranion memory, in which he 

 describes a visit he made to the celebrated Jesse Ramsden 

 in 1780 ; and mentions that he was shown an ingenious 

 mode of elevating a transit instrument by a circle of about 

 3 inches diameter and a level at the eye-end. The vernier 

 fixed and the circle with its attached level movable. To 

 this statement is the sketch of a telescope so fitted, the 

 accompanying portion of w-hich I traced." 



Meridian Mark. — This is an accessory to the transit 

 instrument, so useful and so convenient that it is a matter 

 of surprise that a meridian mark is not more generally 

 provided in connection with transit instruments. It 

 afibrds by day, and, if illuminated, also by night, a means 

 of verifying the meridian adjustment of the transit instru- 

 ment. Fig. 8 represents the meridian mark used in 

 connection with the Bedford Observatory. A plate of 



brass about 4-ioths of an inch thick, 5 inches long, and 

 3 inches wide was fastened by four screws, passing 

 through its corners, to a stone, into which four brass 

 sockets to receive them had been made fast by molten 

 lead. On this plate it was arranged that another of the 

 same thickness should slide ; this was 3* inches long by 

 \\ inches broad, and was attached to the former by dove- 

 tailed side-pieces, and was capable of being adjusted by 

 two long screws pressing against its ends. In the 

 sliding plate there were four slots to receive four capstan- 

 headed screws, by means of which the sliding-plate could 

 be firmlv made fast to the fixed plate after the mark had 

 been duly adjusted to the meridian. This done, the end 

 screws were withdrawn to prevent the possibility of their 



^ If information is needed as 10 hov 

 may be made to Challis's " Lectures c 



this is to be brought about I 

 1 Practical Astronomy," p. 26. 



