BLUE-VIOLET LIGHT IN THE SOLAR CORONA ON AUGUST 30, 1905. 319 



distances measured on pictures I. to Va. as correlative distances on the corona, just as 

 if no diffused light had been illuminating the plate. 



After the experience gained at this eclipse I should again place the picture* as 

 they appear in the diagram, i.e. make the opening of the plate-holder twice as long 

 on a side as the distance between the pictures. This arrangement entails no 

 disadvantage for the short exposures, and for the long exposures the intensity of the 

 diffused light can be eliminated. (See I, last section.) 



(h) Overlapping of Coronas of Neighbouring Pictures. At a point A of the m th 

 picture the intensity of the corona is (i m ) at distance h m , and the intensity of the 

 light which illuminates A for a time t M is (!,) = ,(*.) The same point is illuminated 

 for a time t mil , also by light of intensity (I,, i) belonging to a different part of the 

 corona in the (ml) th photograph, where the corona has an intensity (i m i) and 

 (Ii) = a i (imii)- Point A lies on an equal-blackness curve of the m' h picture, and 

 this curve coincides with an equal-intensity curve of the corona (intensity = i m ) at all 

 points where there is no overlapping. By (c) 



[o.t'.J 1 t m = [a. ()] t m +[a mtl (,)]" .i, 

 or very nearly 



^ = ('.) +(t. i)F... l . 



The equal intensity curve (intensity = i m ) cuts the radial line belonging to A 

 (distance = h) at A', and AA' = AA is the distance of the two curves at A. Hence 



I measured on a diagram the distances of a point A from the solar limbs of the 

 following and preceding pictures, calculated i ml by the formula i = /(/') and thence 

 AA. In deriving the mean distance of an equal-blackness curve [preceding section (d)] 

 I used only those measured distances for which the average value of AA (including 

 AA = 0) is less than a half of the calculated error of the average distance. The 

 number of values is given in Table III. under heading p [see preceding section (t/)]. 

 The average values of AA increase with the accidental error, but they have always 

 the same sign, so that the systematic residuals t; become very small compared with 

 the accidental errors t'. It would of course have been better if all the images had 

 been further apart. (See 1, last section.) 



The outcome of the discussion given in this section is, that the mean corresponding 

 distances given in Table III. are also mean correlative distances on the corona. 



6. Photographs Nos. VI. and VII. 



(a) Duration of Exposure. Owing to the failure in the driving of the plate- 

 holder only two pictures (VI. and VII.) belong to the three exposures 9'02, 2'82, 

 89 - 04 seconds. The sum of the durations of exposure of these pictures is thus given 



