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SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 16 shows two double slides A, B, like a lathe-bed, 155 cm. 

 long and 11 cm. apart, which happened to be available in the author's 

 laboratory ; but single slides at right angles to each other, similar to 

 Rowlandson's, would have been preferable. The carriages C, D, 30 cm. 

 long, kept at a fixed distance apart bj the rod a R b, are in practice a 

 length of -£-in. gas-pipe, swivelled at a and b, 169 "4 cm. apart, and 

 capable of sliding right and left and to and fro, normally to each other. 



The swivelling joint, which functioned excellently, is made very simply 

 of j-in. gas-pipe T's and nipples. The horizontal rod lc fixes the slit 

 standard on the slide S, and is prolonged towards the rear for carrying 

 the flame or Geisler tube apparatus : the rod k also secures the grating 

 standard at t. A large lens at L, of about 56 cm. focal distance, and 

 about 10 cm. in diameter, is placed in front of the grating, and throws 

 an image of the slit S upon the cross-hairs of the eye-piece E. F is a 



