﻿614 The Earl of Berkeley and Mr. D. E. Thomas on a 



gas-tubes ; the Vincent mercury lamp *, M, gives a narrow 

 vertical source of light, which is focussed on the slit, S, of 

 the collimator, C. On the collimator, which contains Wratten 

 niters for isolating the green line, is fixed a metal plate, P, 

 cut so as to have two vertical slits, each 4 mm. wide, with their 

 inner edges 8 mm. apart. Light emerges from the colli 

 mator in two parallel beams, which 



striking a half silvered 



Fig. 1. 



]g= 



& 



mirror, B, are reflected through the jacket, J, to a mirror, N, 

 fully silvered on its front surface. The reflected beams are 

 then brought to a focus by the objective of the telescope, T, 

 a lens of 30 cm. focal length. The interference pattern thus 

 produced is viewed by means of a cylindrical lens of very 

 short focal length. 



Fijr. 2. 



O 



Fig. 2 gives an elevation of the apparatus, omitting the 

 collimator. 



* This lamp was given to us by the Silica Syndicate Co., to whom 

 our thanks are due. The lamp is in the form of a capillary U tube (tore 

 about 1 mm.) made of fused silica — the light is generated in the capillary 

 by heating the mercury there until vapour is formed, it will then run 

 for hours on a direct current (100 volts) of 0"09 amp. Being very small 

 and compact (one form, giving a horizontal source of light, could be 

 carried in a waistcoat pocket), it can be placed exactly where wanted. 



