(270) .* 



[April, 



PROGRESS IN SCIENCE, 



LIGHT. 



Grubb's Automatic Spectroscope. — p, p, p are three prisms placed, we will 

 sav, in the position of minimum deviation for some mean ray — o being the 

 observing telescope, and c the collimator carrying the slit at s. The prism 

 tables are jointed together in a species of chain ; the first point, a, of the chain 

 working on a fixed stud let into the base of the instrument ; the other three 

 joints, b, c, and d, are connected by steel levers with three studs, b', c', d', 

 in a central disc attached at different distances from the centre, and in such 

 positions that the levers form tangents in their mean positions. 



If the distances of the studs from the centre be properly proportioned, the 

 rotation of the disc draws the three joints, b, c, d, in or out as the deviation is 

 required less or more, in just such a ratio as will preserve the position of 

 minimum deviation in all the prisms. So far for the prisms ; and now as to the 

 telescopes. The collimator is stationary, pointed at the centre of the first surface of 

 the first prism, which hardly moves at all. The observing telescope is attached to 

 an arm which is centred on a point, e, in the third prism table, which corresponds 



Fig. 16. 



to the centre of the last surface of the third or last prism, so that in 

 whatsoever manner the telescopes or prisms are turned, this telescope 

 always points towards the centre of that surface, and consequently takes in 

 the whole pencil of light emerging from that surface. On the same pivot is 

 centred also a sector to measure the angle through which the observing 

 telescope is turned. 



The arm to which the observing telescope is attached is coupled to the 

 central disc by a connecting rod, /, /', of such proportions that the movement 

 of the telescope round its pivot for the purpose of viewing the different parts 



