231 PHILOSOPHICAL. TRANSACTIONS. [aNNO 178'2. 



lamps, situated so that the moveable lucid point may pass along the opening left 

 for that purpose. 



Fig. 2 represents part of the arm l, of a larger size ; s the slider ; m the 

 pulley, over which the cord xtyz is returned towards the centre ; v trie other 

 cord going to the pulley n of fig. ] ; r the brass piece moveable on the pin c, to 

 keep the lamp upright. At r is a wire rivetted to the brass piece, on which is 

 held the lamp by a nut and screw. Fig. 3, 4, represent the lamps a, b, with 

 the sliding doors open, to show the situation of the wicks, w is the leaden 

 weight, with a hole d in it, through which the wire r of fig. 2 is to be passed, 

 when the lamp is to be fastened to the slider s. Fig. 5 represents the lamp a 

 with the sliding door shut ; 1 the lucid point ; and ik the openings at the top, 

 and s at the sides for the admission of air. 



Every ingenious artist will soon perceive that the motions of this micrometer 

 are capable of great improvement by the application of wheels and pinions, and 

 other well known mechanical resources ; but, as the principal object is only to 

 be able to adjust the 2 lucid points to the required position and distance, and to 

 keep them there for a few minutes, while the observer goes to measure their dis- 

 tance, it will not be necessary to say more on the subject. 



I am now to show the application of this instrument. It is well-known to 

 opticians, and others, who have been in the habit of using optical instruments, 

 that we can with one eye look into a microscope or telescope, and see an object 

 much magnified, while the naked eye may see a scale on which the magnified 

 picture is thrown. In this manner I have generally determined the power of my 

 telescopes ; and any one who has acquired a facility of taking such observations 

 will very seldom mistake so much as 1 in 50 in determining the power of an in- 

 strument, and that degree of exactness is fully sufficient for the purpose. 



The Newtonian form is admirably adapted to the use of this micrometer ; for 

 the observer stands always erect, and looks in a horizontal direction, though the 

 telescope should be. elevated to the zenith. Besides, his face being turned away 

 from the object to which his telescope is directed, this micrometer may be placed 

 very conveniently, without causing the least obstruction to the view : therefore, 

 when I use this instrument, I put it at 10 feet distance from the left eye, in a 

 line perpendicular to the tube of the telescope, and raise the moveable board to 

 such a height, that the lucid point cf the central lamp may be on a level with 

 the eye. The handles, lilted up, are passed through 2 loops fastened to the 

 tube, just by the observer, so as to be ready for his use. I should observe, that 

 the end of the tube is cut away, so as to leave the left eye entirely free to see the 

 whole micrometer. 



Having now directed the telescope to a double star, I view it with the right 

 eye, and at the same time with the left see it projected on the micrometer : then, 



