L. Bell — Absolute Wave-length of Light. 



353 



Measurement of the Gratings. 



The comparator on which this, the most important portion 

 of the research, was accomplished was the same one described 

 in my previous paper. It had however been improved in sev- 

 eral particulars. The platform carrying the standards had 

 been fitted with smooth rack and screw adjustments, and the 

 microscopes and micrometers were new. The illumination of 

 a grating under the power used, — two hundred and fifty diam- 

 eters — is by no means an easy matter, and at the same time a 

 powerful and symmetrical illumination is absolutely necessary 

 for the most accurate work, particularly in case of rather small 

 grating spaces. I had been thoroughly dissatisfied with the 

 illumination previously used — a lamp at a suitable distance — 

 and now made a radical change. A three candle-power elec- 

 tric lamp was attached directly to the microscope just below 

 the eyepiece and about a foot above the objects measured. A 

 small mirror carried by an arm screwed to the objective re- 

 flected the beam into the Tolles illuminator. A glass bulb 

 filled with water surrounded the light and served the double 

 purpose of stopping radiation and partially condensing the 

 beam upon the mirror above mentioned. 



I am aware that such an arrangement is somewhat revolu- 

 tionary, and it was only after a careful trial that I convinced 

 myself that the heat from so near a source was not injurious. 



In the first place it should be noted that the lamp is only 

 used for a few moments at a time and at intervals long com- 

 pared with the time of observation. Thus the very minute 

 heat wave that reaches the bar through the bulb of water can- 

 not possibly produce a perceptible rise of temperature during 

 the time of an observation, while during the intervals it is 

 completely dissipated. 



As an experimental fact, no heating effect whatever is sensi- 

 ble even after a whole day's observations. To show at once 

 this fact, and the general character of an average series of 

 comparisons I subjoin ten comparisons of DmjS a 2 with a cer- 

 tain decimeter on glass, made at intervals of about three- 

 quarters of an hour on two successive days. The figures are 

 taken directly from my note book. 

 Date. 

 June 1, 1887 Dm,S a 



Jum 





T= 





G + 21 c? -3 



17° 



4 



" + 21-6 



17 



4 



" + 22-1 



IT 



5 



" + 22-1 



IT 



5 



" + 20-8 



IT 



5 



" + 20-1 



IT 



5 



" + 21-4 



IT 







" + 21-0 



IT 







" + 21-0 



IT 







" + 21-0 



IT 



1 



