June 1, 1892.] 



KNOWLEDGE 



113 



a tail composed mainly of five ribbon-like rays, the longest of 

 which measured 35' in length. The rays were equidistant ; 

 probably more would have appeared, but the bright moon- 

 Ught produced a slight general fog on the plate. Cloudy, 

 wet weather followed, and we did not see the comet until 

 the morning of Mai-ch 22nd. Even then clouds were pass- 

 ing, but the air was clearer. A plate was exposed in the 

 star camera, and out of 2h. 30m. we got Ih. .55m. 

 effective exposure ; this photograph shows more of the old 

 rays, one being out to the edge of the plate 70' long, and 

 three new ones not seen before. One of the first five was 

 visible at a distance from the head, bat it could not be 

 seen joining the coma. All these details could be easily 

 seen with a suitable light, but they are too faint to repro- 

 duce photographically ; and the plate used having on it 

 a grating in squares of 5', it was decided to make a careful 

 drawing on an enlarged scale, a process rendered very easy 



Drawing from negative of Swift's Comet. 



by the grating on the negative. It was made by Mr. 

 Sellars, and faith fiilly represented what could be seen on 

 the negative, excepting only the brilliance of the rays, 

 which it was found necessary to make a little brighter in 

 order to get them to photograph. The negative shows a 

 projection of luminous matter to rhe sun, and its gradual 

 turning back to form two of these rays. This and other 

 features have been reproduced in the drawing, and the 

 grating on the negative has made it possible to follow 

 these details. Two of the rays could be seen to the 

 margin of the plate, and probably extended beyond it. 



Every one is familiar now with the selective action of 

 the sensitive plate in regard to the light of coloured stars, 

 and also in that of nebulous light, for, in photographs of 

 nebulae, we find details of light and shade, and a structure 

 shown which is whoUy invisible through the telescope. 

 In this comet photogi'aph, the same action has evidently 

 brought to light ribbon-like rays which would otherwise 

 have remained unknown ; for in the part of the tail visible 

 through the telescope no sign of rays could be detected, 

 even when I knew from the photograph that they were 

 there, and were photographically brighter than the other 

 parts. They are a photographic feature then, and not 

 a visual one in this comet's tail ; or, to put it another way, 

 we have a comet with coloured rays in its tail, probably 

 blue or violet rays from the preponderance of blue in 

 cometary spectre. And the sensitive plate, by its si'lecttve 

 powers as well as by its power of storing up faint light rays, 

 is an aid to vision, and may be used to make a preliminary 

 analysis of cometary or nebulous light, and point the way 

 for more searching analysis by the spectroscope. One 

 morning, through a break in the clouds, we photographed 

 the comet with fifteen minutes' exposure and got rays 

 showing faintly ; at the same time, atmospheric conditions 



were such that we should have photographed an eleventh 

 magnitude star in six minutes. H. C. Bussell. 



Sydney Observatory, ICth April, 1892. 



[The evidence afforded by Dr. Russell's photographs of 

 the bluish tint of certain regions in the tail of this comet 

 is very interesting. It does not necessarily follow that 

 the bluish tint was caused by gaseous incandescence 

 within the streaks, of a different character from that 

 existing in other parts of the comet's tail, or that struc- 

 tures within comets' tails will ever be detected with the 

 spectroscope, for the spectroscope only enables us to detect 

 images due to bright line incandescence and mono- 

 chromatic light. The bluer character of the hght dis- 

 persed by these narrow regions may be (and probably is) 

 due to the finer grain of the particles which disperse the 

 sun's light in the regions comprised within the streaks. 

 It has now been known for some years that the light dis- 

 persed by the tails of comets is partially polarized in a 

 plane passing through the sun, indicating that the greater 

 part of the light is dispersed by particles which are small 

 in diameter compared with the wave-length of light. 

 Some twenty years ago Lord Kayleigh showed that the 

 colour of the light dispersed by tine particles depends on 

 the fineness of the particles," and that the finer the dis- 

 persing dust the richer is the scattered light in short wave- 

 lengths or blue rays. Thus the dust from Krakatoa at 

 first caused the sun to appear red. As the larger particles 

 floating in the air fell to the ground the sun became 

 green, and ultimately it became blue. — A. C. E.vsyakd.] 



THE GLACIAL PERIOD A\D THE PLA^'ET MAES. 

 To the Editor of Knowledge. 



Dear Sir, — It seems to me that one of the chief 

 secondary causes of the Glacial Period has not heretofore 

 been sufficiently enforced. During the short winters 

 and long summers, whatever precipitation occurs will be 

 largely in the form of rain. On the other hand, during 

 the long winters and short summers it will be, on the 

 whole, chiefly in the form of snow. Now the snow, by its 

 great reflective power, wiU cause the earth to lose a very 

 large proportion, perhaps nearly three-quarters, of what- 

 ever radiant energy does fall upon it. Moreover, during 

 the short summer, when the sun is able to melt the snow, 

 there will be an extensive evaporation from its whole 

 surface, forming clouds. These will in their turn reflect 

 away the sun's rays, and at the same time by their shade 

 protect the snow beneath them from melting. 



This cause requires for its action the presence of con- 

 siderable moisture upon the surface of the planet. It has 

 been stated that the glacial theorj' does not appear to apply 

 to Mars. In the case of this planet, however, we have 

 good reason for thinking that it formerly had extensive 

 oceans upon its surface, and the gradual cooling to which 

 it has been subjected has enabled room to be formed 

 for them in its interior ; that is to say, there are probably 

 extensive regions in its interior which are not sufficiently 

 hot to convert water into steam. The water would there- 

 fore, naturally, as is the case with the earth, go under- 

 ground, filling all the microscopic cavities between the 

 rocks. Be that as it may, the appearance of the surface 

 of this planet leads us to think that it possesses at present 

 very much less water in proportion than does our earth. 

 That being the case, there could not be sufficient evapora- 

 tion to form the extensive snow-caps required by the 

 glacial theory. As an illustration of this point, we should 



* See a paper " On the Light of the Sky, its Polarization aad its 

 Colour." bv the Hon. J. W. Strutt (now Lord Ravleigh), in the 

 Pfiil. Mag. for February, 1871, pp. 107120, and " March, 1871, 

 pp. 274-279. 



