unrecognized Wave-lengths. 157 



kindness of Sir William Thomson and Professor Rowland.) 

 First, the short suspending fibre supplied by the makers has 

 been replaced by one 33 centim. in length, stretched and pre- 

 pared 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 deflection of the needle can be read, 

 we have reconstructed the mirror and needles as follows : — 

 For the magnets* soft sheet steel g^ of a millim. thick is 

 rolled up into minute hollow cylinders, each about 8 millim. 

 long and about 1 millim. 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 reflecting mirror 

 is accurately concave, being specially worked for the pur- 

 pose, 9*5 millim. in diameter, 1 metre radius of curvature, 

 weighing 63 milligrammes, and platinized on the front face 

 by the discharge in vacuo of platinum electrodes, by the 

 process of Prof. 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 re- 

 place the aluminum vane of the ordinary instruments by the 

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

 model of lightness 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 metre distance from the mirror, is a portion of a cylinder 

 1 metre in radius, divided into 500 divisions of 1 millim. each. 

 The optical arrangements for illuminating and forming an 

 image of the wire form one of such precision that a motion 

 of -j^ of one of these divisions can be distinctly noted. There 

 is an independent 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 ordinary construc- 

 tion, directed onto a flat attached to the needles, but in which 

 the concave mirror, already described, becomes the mirror of 

 a Herschelian telescope itself. Ordinarily the condition of 

 astaticism of the needle is such that, without any damping- 

 magnet, it will execute 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, cor- 

 responding advantage. 



* The design and construction of the hollow magnets is due to Mr. F. 

 W. Very of this Observatory. 



t Prof. Wright has had the goodness to platinize these delicate mirrors 

 for us himself. 



