278 C. Ba/rus — Method for the Observation of* Coronas. 



seeing that the eves may be placed all but in contact with the 

 fog chamber. 



2. Apparatus. — Figure lb and d, the latter drawn to scale, 

 show a general disposition of the apparatus. S' and S" are 

 the two circular sources of light lying in the same horizontal, 

 and movable in opposite directions in equal amounts, at the 

 control of the observer sitting near at the fog chamber F. 

 .&' and S" are therefore always symmetrical with respect to the 

 vertical plane SR. The diffraction of rays due to the fog 

 particles a, b, c, in F, produce coronas seen at nn' and n'n", 

 and the lamps S', S" have been adjusted at a distance S, so 

 that the selected annuli of the coronas are in contact at n'. The 

 angular radii of the coronas, marked 6 or shaded in the diagram, 

 are nearly equal '2R tan 6 = S, where R is the distance of the 

 axis of the fog chamber from the track 8 in parallel with it. 



On a double track at S, the two carriages for the lamps, 

 S', S" are moved with sprocket and chain or in a similar 

 manner and provided with a scale stretched between them, 

 reading to centimeters. This is a lath of wood about 3 meters 

 long with one end fastened at /S', the other free, while the scale 

 moves across an index at /S". A pole at R, with the end in the 

 observer's hand, moves the two central sprockets and at the 

 same time serves for the measurement of R, should this 

 accidentally vary. 



3. Errors. — The figure shows clearly that the angle of 

 diffraction corresponding to the fog particles nearer and far- 

 ther from the eye will not be the same and that this effect will 

 vanish as the coronas are smaller, as the diameter or thickness 

 of the fog chamber is less, and as the distance R from the 

 source is greater. Slightly different annuli overlap ; but the 

 effect is much less here than in the case of a single source, 

 where the active fog particles lie oblique to the axis. See 

 figure la and figure lb at «, b, c. In practice this effect is 

 probably negligible if the dimensions of apparatus and disposi- 

 tion of parts are properly chosen, particularly so since the fog- 

 particles themselves are not usually so nearly of a size as to 

 imply less overlapping. In fact the true corona, if large or 

 even of moderate size, is seen but for an instant immediately 

 after exhaustion. It therefore shrinks rapidly, as may be 

 gathered from the following incidental data given in figure 2 

 and obtained with fog particles originally about •00002 cm in 

 diameter, belonging to the large yellow-blue corona. 



The coronas shrink as the fog particles increase in number 

 and decrease in size at an accelerated rate. This shrinkage 

 begins as soon as the corona appears. The initial rates must 

 be estimated at a decrement of number greater than l - -i per 

 cent per second, supposing that no water is added from other 



