56 



TRANSACTIONS OF THE 



[Nov. 16, 



e results and this substituted in the above formula yields X. 

 The colors produced between crossed nicols are : 



V. Traube's Device for Shutting out Extraneous Light 

 During the Measurement of Crystals.* 



The apparatus shown in Figs. 7 and 8 is attached to the col- 

 limator tube easily yet firmly by the hemi cylinder a which clips 

 the tube just below its centre so that the ring c fits snugly like 

 a cap over the end of the collimator. A conical tube d connects 

 G with the small black cylinder e which surrounds the crystal 

 on the side except at the window/, but is open above and below. 



Fig. 7. 



Fig. 8. 



The light from the lamp passes through the collimator and its 

 prolongation the conical tube d to the crystal and is reflected to 

 the observation telescope. The cylinder e shuts out much of the 

 extraneous light giving a darker field in which the luminous 

 signal is b}^ contrast brighter. Still more light can be shut out 

 by laying a card over the top of e or better by the cap g fig. 8, 

 which may be turned to diminish the window/ to a mere slit. 



*Eine einfache Verdunklungsvorrichtung fvir das Goniometer rait horizontalem 

 Theilkreis. Xenes .lahrbudi fiir Mineralogie, etc. 18i)4. Bd. IT. 



