BLUE -VIOLET LIGHT IN THE SOLAR CORONA ON AUGUST 30, 1905. 315 



halves of an equal-blackness curve coincide each with an equal-intensity curve of the 

 corona, though not with the same curve of the corona, and that on the two halves of 

 Photographs 1. to IV. an equal-blackness curve differs inappreciably from an- equal- 

 intensity curve ; (2) that on account of the overlapping of the coronas belonging to 

 neighbouring pictures the intensity and blackness curves do not coincide at certain 

 position -angles. Therefore, if the mean distance of each equal-blackness curve be 

 derived separately for each half of Photographs V., VI. and VII., and all measure- 

 ments be excluded which belong to position-angles where there is appreciable 

 overlapping of coronas, the mean distance will also be the mean distance of an equal- 

 intensity curve. Each measurement belonging to a position-angle p, and reduced in 

 accordance with (a) and (6), plus the correction oh derived above (c), gives a mean 

 distance of the equal-blackness curve, and there are as many values of this mean 

 distance as there are measurements. Their average value h is the final value, and 

 its error can be determined from the differences from the mean. Table III. contains h, 

 its error and the number of measurements p which contribute to the mean value. 

 p = 24 indicates that all the points of the curve, position-angles to 345, were used. 

 For p = 21, the points at position-angles 225, 240, 255 degrees are excluded and for 

 p = 19 those at 210 and 270 degrees are omitted in addition. For Photographs 

 V6. to VII. the points omitted lie symmetrically round position-angles 60 degrees 

 and 240 degrees. The quantities are given in unit of 10~ 3 solar diameter, those 

 derived in inches being multiplied by 1'852 (diameter of moon on photograph 0'565 

 of an inch, diameters of moon and sun 994'5 and 9507 seconds). I designate by 

 " corresponding distances " the distances from the sun's limb of two points on two 

 different pictures of the corona at which there is equal blackness. In Table III. the 

 mean corresponding distances stand on the same line. I shall show in the next section 

 that at these tabulated distances the ratio of the intensities of the corona is a constant 

 for each two photographs. 



5. Correlative Distances on Corona. [Definition see under (</).] 



(a) I employ the following notation. 1 or S is an intensity of light acting on a 

 photographic plate, and they are the quantities of light falling on unit area of the 

 plate, which is the area cut out in the focal plane of the camera by unit of spherical 

 angle at the centre of the object-glass, i or s is an intensity of a luminous object, 

 i.e., a quantity of light falling from unit area of object (area cut out by unit of 

 spherical angle) on unit area of the object-glass, which unit area equals that for the 

 plate, a designates the exposed area of the object-glass and t the time during which 

 the plate is illuminated. Then I = as and S = as. 



For the pattern of screens by which I reduced the aperture of the lens the loss of 

 light due to the object-glass will be about proportional to the aperture, and it need 

 not be taken into account, but the effects of diffraction require special investigation 

 (see Appendix I.). 



2 s 2 



