48 Conductivities and Viscosities in Pure and in Mixed Solvents. 



building. There is every reason to suppose that the scale could be 

 read to about 0.25 mm. when the building was quiet. Duplicate 

 readings under ordinary conditions nearly always agreed to within 

 0.5 mm. 



THE SPECTROSCOPE. 



The grating spectroscope was designed and built in the Physical 

 Laboratory under the direction of Professor John A. Anderson. The 

 use of such a spectroscope presents two especially desirable advantages. 

 First, the position of regions of absorption can be very accurately 

 determined; second, due to its high resolving power, the structure of 

 even the very narrow absorption bands and some absorption lines can 

 be studied in detail. The spectroscope is so constructed that either the 

 photographic plate or any of the radiometric instruments can be used 

 with it. The use of the radiomicrometer enables us to determine not 

 only the actual positions of regions of absorption, but also to make 

 quantitative measurements of the light transmitted by a solution for 

 a wide range of wave-lengths. The deflections of the radiomicrometer 

 give an accurate measure of the relative intensities of the different 

 absorption bands, and of the different parts of the same bands. It 

 will thus be seen that the radiometric method has distinct advantages 

 over the photographic, which is chiefly useful in determining the 

 positions of regions of absorption and the general characteristics of the 

 visible spectrum as transmitted by the solutions. Moreover, the photo- 

 graphic plate is only sensitive from X = 0.2ju to X = 0.76ju, whereas, with 

 the radiomicrometer and the apparatus used in this work, quantitative 

 measurements of absorption could be made from X = OA/j. to X = 2.0ju, 

 using slits only 1 mm. in width. Considering the high dispersion that 

 could be obtained with the 4-inch grating, this width of slit gives a 

 very pure spectrum. 



The plane 4-inch grating with which the spectroscope is equipped 

 was ruled by Anderson; and the ruling is of such a character that a 

 very bright first-order spectrum is produced. This is quite essential 

 for radiometric measurements in the visible region of the spectrum. 

 Energy measurements were made in both first-order spectra, one being 

 on each side of the central image. The spectrum on one side was found 

 to be somewhat more intense, and therefore this brighter side was used. 



If either the intensity of the light source or the sensitiveness of the 

 radiomicrometer exceeds certain limits, it is possible that light energy 

 from the second-order spectrum will vitiate the energy measurements 

 made in the first-order spectrum. This will be readily seen if we con- 

 sider that wave-length of light X = 0.35,u of the second order overlaps 

 wave-length of light X = 0.7jU of the first order, etc. Therefore, it 

 was desired to know if, with a Nernst glower burning at 0.8 ampere 

 and 120 volts, the second-order spectrum had sufficient energy to be 



