ON SPECTROSCOPIC MEASUREMENTS. 



175 



from which the second column in the following table was calculated. 



<t>/<po 







1 



2 

 3 

 4 

 5 

 6 

 7 

 8 

 9 

 



From this table it appears that if the visibility is to be estimated by 

 observations with a telescope of 12 mm. aperture (or with a circular 

 aperture about one-fourth greater) an error in the adjustment of 

 the surfaces of a second of arc would produce a diminution of 4 or 5 

 per cent, in the visibility. Accordingly, if the ways on which the mirror 

 carriage moves are not true to this degree, it is necessary to make the 

 adjustment for every observation. 



This can be done with very great accuracy by moving the beam of 

 light from side to side and adjusting the mirror until there is no percep- 

 tible alteration in the size of the rings. Since the admissible error in 

 adjustment is inversely proportional to the aperture, the observations may 

 be facilitated by making this as small as possible if there be light to 

 spare. This is all the more necessary for the same reasons, if the sur- 

 faces be not true. However, the error due to this source may be easily 

 corrected (since all the observations are affected alike) by multiplying by 

 a constant factor. 



In order that the visibility curve may extend as far as possible, it is 

 necessary that the vapour should be very rare. Accordingly, in all but a 

 few cases to be mentioned later, the substance to be investigated was in- 

 closed in a vacuum tube which was previously heated to drive off any 

 moisture or occluded gases. 



The vapour was rendered luminous by the discharge from the second- 

 ary of a large induction coil, whose primary current was interrupted by a 

 rotary break attached to the armature of an electric motor, making 

 about twenty to thirty breaks per second. The steadiness of the light 

 thus obtained was far greater than with the ordinary Foucault inter- 

 rupter. Probably it would have been still more satisfactory to use an 

 alternating dynamo properly wound to give a strong current with com- 

 paratively few alternations. 



The box surrounding the vacuum tube was heated just sufficiently to 

 give a steady bright light, and the temperature then kept as nearly uni- 

 form as possible. This temperature was usually taken to represent that 

 of the vapour within the tube. This is, of course, only a rough approxima- 

 tion to the truth ; and in some cases the estimate was much too low. 



As it was not intended to include in the present work an elaborate 

 study of the effect of temperature, this matter was not of great conse- 

 quence. It may be suggested, however, that a very much closer approxi- 

 mation to the real temperature could be obtained by winding a platinum 

 wire about the capillary portion of the tube and deducing the tempera- 

 ture from the variation of its resistance. A preliminary experiment in 

 which a platinum wire passing through the tube and heated by a current 



