'26S Mr. T. H. Blakesley on a Direct- Vision 



fact I have established more clearly in a paper recently read 

 before the Royal Society. This permanent part of the leak 

 is no doubt due to ionization of the air in contact with the 

 hot platinum, and it is in no way analogous to a mono- 

 molecular chemical reaction involving only a small quantity 

 of material. 



Mr. Richardson has also {loc. cit.) observed this permanent 

 part of the leak as well as the temporary part which decays 

 comparatively slowly. The positive leak from hot platinum 

 can therefore be described as consisting of three parts : — 



(1) A temporary part disappearing in a few minutes, and 

 apparently independent of the air-pressure. 



(2) A temporary part disappearing after many hours' 

 heating. This part may be due to the same cause as (3), and 

 its decay may be due to a gradual alteration in the state 

 of molecular aggregation of the platinum by long continued 

 heating. 



(3) A permanent part which is zero in a vacuum and 

 increases wdth the gas-pressure, and is at any pressure a 

 definite function of the temperature. This part is due to 

 ionization of the air in contact with the platinum. 



I found {loc. cit.) that (1) reappears on leaving the platinum 

 in air ; Mr. Richardson has confirmed this result. 



It appears from the results obtained that (1) is due to 

 the presence of some substance in the platinum which 

 escapes at a rate proportional to the leak, so that (I) may 

 be correctly described as analogous to a monomolecular 

 reaction. 



Yours very truly, 



H. A. Wilson. 



XXXIII. Direct- Vision Spectroscope of one kind of Glass. 

 By T. H. Blakesley, M.A.* 



THE fundamental j^rinciple underlying the action of this 

 instrument is the following : — Starting with the ordinary 

 formula connecting ju, the index, A the refracting angle of a 

 prism, and D the minimum deviation — 



. A . A+0 

 /i . sm ^ = sm — ;^ — 



if the angle A is so chosen that the de^dation shall be equal 

 * Comnumicated by the Plij'sical Society : read May 8, 1903. 



