44 Proceedings of the Royal Irish Academy. 



Recording apparatus. The recording apparatus is a modification of 

 that deyised some years since by Mr. Grubh, and found by those 

 who have employed it to combine a very considerable amount of 

 accuracy with great facility in use, dispensing also with the necessity 

 for a reading lamp at the time of observation, and thereby preventing 

 that loss of time which is inevitable if micrometric apparatus of the 

 ordinary construction be employed for the measurement of extremely 

 faint spectra. 



The bar m (Fig, 1.) is thickened and split at one extremity to 

 receive the end a" of the axis on which the base-board carrying the 

 telescopes moves. 



The two prongs of the fork thus formed can be brought together 

 so as to grip a" tightly, by means of a small capstan-headed screw. 



The link n connects the free end of 711 with the recording arm i. 

 The two ends of n are perforated with square holes, the diagonals of the 

 squares being longitudinal and transverse to the link. 



Through these square holes pass screws into m and i, in which 

 they are fixed. Owing to the position of the holes, these pins 

 work practically in V bearings, and steadiness of movement is thereby 

 secured, 



i moves on a fulcrum at /, and carries near its free extremity a 

 pin p, directed inwards towards the slider. By pressing the finger 

 gently on the outer side of the arm i near the end, a clot may be made 

 on a card, and a record thereby made of the position of the arm at the 

 time. The slider moves in either direction readily, parallel to the 

 line Jill. If m be changed to a' , and the telescopes be raised, m will 

 be constrained to rise also, and by means of the link n will raise i 

 and p with it. 



Since the distance from the object glass of the telescope to the 

 final image is 9 in., m being 4 in. long, nf \^ in., and fp 15 in., 

 any motion of the pointer, if small, will be multiplied at p by the 

 improper fraction 



4 X 15 , , 

 = 4-4 ; 



9 X 1-5 



an amplification which is found to be practically sufficient. 



The slit is formed of two long long brass rules, jointed together 

 in exactly the same manner as for a parallel ruler. 



The links are perforated at theii' centres for two screws, one for 

 each, on which they severally turn. To each extremity of the lower 

 rule is fastened one end of a piece of flat linked chain, the central 

 portion of the chain being coiled twice about a spindle, on which is 

 fixed a lever about 6 inches in length. 



By altering the position of this lever, the chain is di'awn one way 

 or the other, and causes the rules to move. 



As the axes on which the links turn are situated in the centre of 

 their length, the approach of the jaws of the slit to a line between. 



