STANDARDIZATION AND EFFICIENCY OF THE FOG CHAMBER. 31 



number z of the exhaustion. The curves are again linear in character, 

 but in case of the outer edge s'" ', the insufficient size of my fog chambers 

 and the greater vagueness of definition interferes with close measurement. 

 The results are 



Part VIII: s' = i. 00 + 1.07 (18-2) 5"' = 3.oo + 1.93 (i8-z) 



5 = 1.61 + I - I 4 (182) 



Part VII: $' = 0.50+1.13(17-2) s'" = 1.50+ 2.13 (17 -z) 



5 = 1.08 + 1.20 (17 z] 



As a rule s'" is less than twice 5'. The data for s f and 5 are obtained 

 alternately, as the table shows, the green color passing from the disk of 

 the first ring and returning again in succession. 



/ 



26. Inferences. Interference and diffraction. The tables and charts 

 show, in the first place, that the disk and first ring of the coronas 

 are alternately vivid green, other colors being dull because the remaining 

 lines of the mercury spectrum are faint. At intervals neither disk nor 

 ring are quite green. Hence there is a periodic term impressed on the 

 diffractions, which may be identified as an interference similar to the 

 case of the lamellar grating referred to in another paper.* 



When monochromatic light is used it is necessary, therefore, to observe 

 both the edge of the disk and the inner edge of the first ring, for neither 

 appear vividly at the same time. The chord 5 on a radius of 30 cm., for 

 the minimum in terms of the corresponding chords, s' and s", of the first 

 and second edges specified, may then be written 



5 = 0.55+1065' 5=-o. 50+935' 



Probably the ratio s/s' and s js" should be constant and the absolute 

 term in these equations is an error of observation; but as it is small 

 little depends upon it, millimeters only being significant. 



If we summarize all the observations for dsjdt, the agreement as a 

 whole is in keeping with the nature of the observations and reasonably 

 satisfactory. Thus in 



Series I, II, ds'/dz = o.<)o s = 0.44+ 0.855' (goniometer in front) 

 HI, 1.15 



IV, 1.13 



V, 1. 10 



VI, 1.13 



VII, 1.13 



VIII, 1.07 



5 = 0.55 + i -065' (goniometer behind) 

 mean ds f jdz =1.12 



*Amer. Jour. Sci., xxv, p. 224, 1908. 



