96G Mr. T. II. Blakesley on the Diameter of 



temperatures at which small quantities ai* radium emanation, 

 when once condensed, will volatilize from the surfaces of 

 glass and of metals. With this flow method of experiment, 

 using quantities of emanation varying between the equili- 

 brium amounts from "2 x 10 -4 and 2x 10~ 3 mom. of radium, 

 and with a rate of rise of temperature of about o, 4: C. per 

 minute, it was found that there is a slight and gradually 

 increasing volatilization until the temperature approaches 

 — 160° (<. Approaching this temperature, the volatilization 

 becomes relatively very rapid, and above it volatilization 

 practically ceases. 



The temperature, — 160° C, mentioned above, is merely 

 the final temperature of separation of the emanation from the 

 condensing surfaces in the experiments ; its measurement 

 serves as a method o( comparing the effecl of the various 

 surfaces mentioned on the volatilization. This temperature 

 i- some degrees lower than the majority of results in the 

 original experiments of Rutherford and Soddy, where the 

 temperatures measured were the average temperatures over 

 the whole of the condensing spiral, and where there was a 

 higher rate of rise of temperature. 



It is not to be understood that it is impossible for emana- 

 tion to remain condensed on a metallic or glass surface at 

 temperatures higher than —160° C. ; for further e.xperi- 

 ments by another method have shown that the temperature 

 of final separation from the surface depends, as would be 

 expected, on the quantity of emanation condensed. These 

 experiments will he described in a later paper. 



The writer is glad to acknowledge jjreat kindness and 

 assistance on the part of Prof, Rutherford. 



University of Manchester, 

 August 1910. 



CVIII. A Means of Mea.su ring the Apparent Diameter of the 

 Pupil of the Rye, in very feeble Light. By T. H. 

 Blakeslf.y *. 



TIIHE advantage obtained by the magnifying power of a 

 I telescope or a microscope may be very seriously 

 diminished if the orifice of the eyepiece through which the 

 light issues to the eye is so small as not to fill the pupil with 

 light. The magnification is inversely proportional to the 

 focal length of the eyepiece, and therefore to the orifice 

 with which it is furnished, which therefore has a smaller 



* Communicated bv the Author. 



