628 



NATURE 



\pct. 30, 1879 



original book, perhaps into a sixth. As to the need of such an 

 index there cannot be two opinions. If, however, a fuller 

 classified catalogue, such as is proposed by Mr. Garnett, be 

 thought necessary, would it not be better to make it in the form 

 of a series of indexes of separate subjects ? The day for great 

 encyclopaedic works is nearly past, and as the astronomer cares 

 little for the papers of the zoologist, and would find them only 

 in his way, so both the zoologist and the astronomer would wish 

 to have his own subject in a distinct volume. 



This leads me to the chief point of this letter, which is to draw 

 attention to the work that is already being done. I have received 

 a letter from Prof. Holden, of the United States Naval Observa- 



tory, in which he announces to me, as Secretary of the Index 

 Society, his intention of making an index to all the entries re- 

 ferring to astronomy in the Catalogue of Scientific Papers, and 

 also informs me that Prof. Abbe, of the United States Signal 

 Service and Weather Bureau, has a complete card catalogue of the 

 meteorological entries in the Royal Society Catalogue. Probably 

 other workers have done the same for other subjects. This is, I 

 think, the best use to make of the Catalogue of Scientific Papers, 

 which is of immense value, in the first place as a catalogue of 

 authors, and secondly, as a collection of authentic documents 

 from which a series of subject-indexes may be drawn. 

 Society of Arts, October 28 Henry B. Wheatley 



Solar Halo 

 I INCLOSE a sketch of a remarkable solar halo and parhelia 

 'which I observed here on the 22nd ult. As I happened to have 

 facilities at hand, I was enabled to take the dimensions and 

 position of the various features of the phenomenon with stiffi- 

 cient accuracy. These appearances were first seen by me at 

 9.30 A.M., and continued nearly constant in brilliancy till about 

 10 A.M., when they gradually faded, and at 10.30 hardly any- 

 thing was visible except the outer ring, abefg, which continued 

 till I P.M. At 9.45 the inclosed sketch was made and the 

 measurements taken. Tlie sun had then an altitude of about 30", 



and was very misty' and indistinct. It was surrounded by an 

 ordinary solar halo of about 45* diameter ; and through the sun 

 passed another perfectly unbroken circle whose centre was 

 exactly at the zenith. This circle had no colour and was similar 

 in character to the ordinary concentric solar halo ; its angular 

 diameter was 1 20° ; concentric 'with this was another circle of 

 78° diameter. This inner circle was not quite perfect at the 

 point where the circle, abed, touched it ; it was slightly dis- 

 torted, and through the same point {d) passed a portion of 

 another circle of larger radius, j k. The junction of these three 

 circles formed a beautiful spectrum, and was the most brilliant 



part of the whole phenomenon. The inner circle was also im- 

 perfect on the side opposite the sun, when it branched off in two 

 opposite curved tails, which, crossing the outer circle at the 

 same spot, g, formed a mock sun. Two other mock suns were 

 to be seen at e and/, 50° to each side of g. These three mock 

 suns were all colourless, but at a and b, not on the concentric 

 halo, but about 2° outside it, were two beautifully coloured 

 mock suns, each being a perfect spectrum. 



Finally, two portions of coloured circles were seen at / and m, 

 with the concave side towards the sun, and two others at « and 0, 

 with their convex sides towards the sun, but in every case (both 

 of circles and mock suns, a and b) the red colour was nearest the 

 sun and the blue farthest from it. 



I may mention, to give an idea of the brilliancy of the pheno- 

 menon, that many persons mistook one or other of the coloured 

 bands for rainbows, and in one case one of the mock suns was 

 supposed to be the sun itself (the sun happening to be hid from 

 the observer by some adjacent buildings). 



Dublin, October ii Howard Grubb 



\ Karl Friedrich Mohr 



In your notice on the late Karl Friedrich Mohr there is no 

 mention made of one of his most remarkable works, " Die 

 Geschichte der Erde," the first edition of which appeared in 

 1866. In it he takes up what he considers entirely new ground 



