324 Mr. Gassiot Description of a Riff id Spectroscope. [June 15, 



to endeavour to determine the temperature corrections which it will be 

 necessary to apply to the results obtained by the instrument, and also to 

 find if the line resumed its position exactly after the instrument had been 

 subjected to considerable changes of temperature, or carried from place to 

 place. From Tables III. and IV. it may be noted that the readings have 

 a constant downward tendency. To endeavour to account for this retro- 

 gression, I can only venture to hazard the suggestion that the index of 

 refraction of the glass of which the prisms are made may be slightly 

 varying from some change due to annealing not having yet been entirely 

 completed. 



Although showing generally a downward tendency, the decline is not 

 quite uniform, and there are some apparent discrepancies in the readings ; 

 these may be errors of observation, arising principally from differences of 

 intensity in the source of illumination. Such differences tend to alter the 

 width of the bright line observed ; and as one of its edges is taken for the 

 point of measurement, the position of the line is apparently changed. 

 Mr. Stewart is, however, of opinion that these variations are so slight that 

 they will not be likely to interfere with the instrument being used for the 

 purpose for which it was designed. 



TABLE I. 



Readings of one of the D-lines taken with the Rigid Spectroscope at the 

 Minories. 



Temp. Temp. Difference in 



Date. Air. Prism. Micrometer. divisions. 



February 1 57'5 52'5 5-521 



Ioo.n oo.r /?.*Ti- i * ***-' 







2. 

 .3. 



6. 

 6. 



RQ.A CQ.A K.AC ~\ 



1-18 



Q O^.A o r . r /? . i A f 



11 57-5 52-5 4-85 



Results from 6th to 1 Hh : 



Variation in temperature correction for a change of temperature 



of 30 O-OOO/ inch. 



Separation of D-lines 0'004 inch. 



