November 4, 1922] 



NA TURE 



605 



of horizon that is effective is about two miles, at 

 which the change of colour is just visible. The " second 

 phenomenon " of Prof. Porter is the only one which 

 I have seen with open eyes. 



W. M. Flinders Petrie. 



A Broadcast " Rainbow." 



On Friday, September i6, I witnessed an atmo- 

 spheric phenomenon sufficiently unusual, I believe, 

 to merit a record. Standing on Ogmore Down near 

 Bridgend in this county (Glamorgan) at 2.30 p.m. and 

 looking northwards across the broad vale towards 

 the Maesteg hills, there appeared to me a broadcast 

 rainbow colouring, stretching east and west for several 

 miles along the vale. The day was exceptionally 

 fine, with brilliant visibility and no trace of mist. 

 The clouds were small and scattered, with a distant 

 bank of cumulus beyond the hills, while the colours 

 were clear and unmistakable, covering, from red in 

 the west to blue in the east, an angle of about fifty 

 degrees. The height of my point of view was about 

 300 ft. above the sea, and the whole apparition hung, 

 like a veil of pure, immaterial colour, at about the 

 level of my eyes, covering the distant hills but without 

 screening their smallest particular. R. C. McLean. 



The Botanical Department, 

 University College, 



Cardiff, September 19. 



Colour Observations of the Moon. 



I wish to place on record a few observations I 

 have made of the lunar surface seen through colour 

 filters, which point to the presence of coloured light 

 of various shades reflected from different parts of 

 the moon. 



The light values of various points of the moon's 

 surface were fixed by means of a photometer shaded 

 by colour-screens, corresponding to similar screens 

 fixed to the eye-piece of the telescope. 



Owing to the two different sources of light, electric 

 light in one case and the lunar rays in the other, 

 and also owing to atmospheric changes, the two 

 sets of filters had to vary considerably in colour, in 

 order to produce the identical colour impression on 

 the eye. 



For the telescopic eye-piece I used the following 

 colour screens throughout these observations : 



1. Red of approx. 6563 " Tenth - metre " wave-length 



(absorbing practically all but 

 red rays and a little yellow) . 



2. Green ,, 5173 wave-length (absorbing all but 



green and blue rays). 



3. Violet ,, 4420 wave-length (only absorbing 



yellow rays). 



For the photometer I used varying colours (accord- 

 ing to atmospheric conditions), averaging 



i« of approx. 6120 wave-length. 

 2a „ 4922 



3« .. 455° 



The area examined was the vicinity of Clavius. 

 I determined the light value of the brightest spot in 

 the neighbourhood (A) through the three screens, 

 and in the same way a number of other spots ; B, 

 the eastern inside wall of Clavius ; C and D, the 

 floor of the crater plain ; E, the great valley to the 

 west of Clavius ; F, the southern slopes of Maginus. 



The result of the two observations, No. I. on 

 August 31, No. II. on October 1, 1922, was as follows, 



reducing the figures to percentages, the brightest 

 spot (A) being taken as 100 per cent, in each case : 



In examining the above list it will be noticed that 

 there is a discrepancy regarding the point B, which 

 may be due to the dazzling brilliancy of this area 

 during the first observation. All the remaining 

 figures, however, agree remarkably well, considering 

 the extreme simplicity of the instruments I employed. 



The experiments tend to show that (1) the floor 

 of Clavius (also of Longomontanus, which I observed 

 on October 1) is of a mauve colour. These areas 

 may be basaltic and not unlike some Hawaiian 

 crater plains of solid lavas. (2) The Terra Photo- 

 graphica to the west of Clavius has most probably 

 a mottled surface of brown areas on a blue back- 

 ground such as copper sulphate. (3) The southern 

 slopes of Maginus are intense green, probably streaked 

 with areas of the same substance and colour of the 

 crater valleys recorded. 



It would be of great interest to survey accurately 

 in this manner districts such as the crater Linne. 

 Later tests would then clearly show if any change 

 could be recorded in these debated areas, at least 

 so far as coloration is concerned. A. F. Warth. 



109 Sandford Road, 



Moseley, Birmingham. 



NO. 2766, VOL. I io] 



The Local Handbook of the British Association. 



In Nature of October 21, p. 539, a reviewer states 

 that " the ideal handbook in connexion with the 

 annual visits of the British Association has yet to be 

 written." A Committee of the Association might 

 well be appointed to draw up a scheme. Meanwhile, 

 I venture to offer the following suggestions : 



1. The book should above all be portable, and for 

 this purpose it should be divided into separate 

 pamphlets, each tucked under a separate tape in a 

 cloth-covered binding, common to the series. One 

 could then select one's pamphlet and leave the rest 

 in one's hotel. 



2. Every handbook should have a complete index 

 for facilitating rapid consultation. 



3. There should be a map of the district on the 

 half-inch or quarter-inch scale. 



4. There should be a geological map. 



5. The compilers should take a lesson from 

 Baedeker and give practical details : population, rail- 

 way stations, hotels, cab-fares, post office ; short list 

 of the chief features — museums, art galleries, 

 libraries, churches of architectural interest, and other 

 guide-book information— in two or three pages, with 

 street in which situated, times of opening, etc., with 

 an asterisk indicating the most noteworthy. 



6. Unless the handbook can be sent by post in 

 advance to members (they might pay the postage) it 

 is of little use writing long-winded articles, as visitors — 

 attending sections in the morning, scientific excursions 

 in the afternoon, and addresses in the evening— have 

 no time to read them. Bernard Hobson. 



Thornton, Hallamgate Road, Sheffield, 

 October 20. 



T 2 



