﻿S. P. Langley — Unrecognized Wave-lengths. 91 



indebted for suggestions due to the great kindness of Sir 

 William Thomson and Professor Kowland.) First, the short 

 suspending fiber supplied by the makers has been replaced by 

 one 33 cm in length, stretched and prepared with particular 

 care. Next, since the effect of a given minute change of current 

 is proportionable (other things being equal) to the magnetic 

 moment, and to the minuteness with which the angular deflec- 

 tion of the needle can be read, we have reconstructed the 

 mirror and needles as follows. For the magnets* soft sheet 

 steel -£-§ of a mm. thick is rolled up into minute hollow cylin- 

 ders each about 8 mm long and about l mm diameter. These 

 are hardened and made to take a permanent charge of nearly 

 900 Gaussian units. Ten of these are placed behind the back 

 of the mirror and ten below, making twenty in all. The reflect- 

 ing mirror is accurately concave, being specially worked for 

 the purpose, 9*5 mm in diameter, 1 meter radius of curvature, 

 weighing 63 milligrams and platinized on the front face by the 

 discharge in vacuo of platinum electrodes, through the process 

 of Professor Wrightf of Yale College. The stem which unites 

 the upper and lower system of the magnets is a hair-like and 

 hollow tube of glass, while it occurred to me to replace the 

 aluminum vane of the ordinary instruments, by the wings of a 

 dragon fly (Libellula), in which nature offers a model of light- 

 ness and rigidity quite inimitable by art. 



The glass plate which encloses the front of the galvanometer 

 has optically plane and parallel sides, and the screen, placed at 

 1 meter distance from the mirror, is a portion of a cylinder 1 

 meter in radius, divided into 500 divisions of l mm each. The 

 optical arrangements for illuminating and forming an image of 

 the wire form one of such precision that a motion of y 1 ^ of 

 one of these divisions can be distinctly noted. There is an in- 

 dependent provision by means of which the image of a second 

 opaque and inverted scale can be viewed by the observer 

 through a telescope, not as in the ordina^ construction, directed 

 on to a flat, attached to the needles, but in which the concave 

 mirror, already described, becomes itself the mirror of a Her- 

 schelian telescope. Ordinarily the condition of astaticism of the 

 needle is such that without any damping magnet, it will exe- 

 cute a single vibration in not less than 15 nor more than 30 

 seconds. Much greater sensitiveness can be given to it, of 

 course, but without, as we have found, corresponding advan- 

 tage. 



For the purpose of forming an estimate of the sensitiveness 

 of the instrument, it may be stated that, when making a 



* The design aud construction of the hollow magnets is due to Mr. F. W. "Very,, 

 of this Observatory. 



f Professor Wright has had the goodness to platinize these delicate mirrors for 

 us himself. 



