ON FIXING A STANDARD OF WHITE LIGHT. 119 



well as the large and numerous artificial rockeries in gentlemen's grounds 

 in the district, show that they are not without value, and have been very 

 largely utilised. Indeed, it is to be feared that, unless care be taken to 

 prevent it, those now remaining in the spots they have so long occupied 

 undisturbed, may become rapidly fewer, and disappear altogether at no 



distant date. 



It must be understood that in the foregoing remarks 1 have conhned 

 myself to the limited district I visited. Mr. Fox told me that he had 

 noticed them elsewhere, and especially near Maristowe, about 3 miles off 

 as the crow flies, in a N.N.W. direction. 



The Committee have confined their Report to a simple record of facts, 

 without attempting to decide how far these facts support any special 

 theories. It is believed that many other erratic blocks hitherto unrecorded 

 are scattered over England, Wales, and Ireland ; and that every year a 

 large number are destroyed by agriculturists and builders. The Com- 

 mittee appeal, therefore, to local observers to report upon them in order 

 that evidence so valuable with respect to many problems of the glacial 

 epoch may be preserved. 



I 



Report of the Committee, consisting of Captain Abney, Professor 

 W. G-. Adams, and Professor G. Carey Foster, appointed to 

 carry out an Investigation for the purpose of fixing a Standard 

 of White Light. Drawn up by Captain Abney {Secretaryy 



Since the last meeting of the British Association a large number of 

 experiments have been made with various lights, in order to ascertain the 

 constancy of the various component radiations, the total qtiantity of such 

 radiation having been only partially examined. Amongst others that may 

 be mentioned are coal gas and the ordinary sperm candle. The former fails 

 to satisfy the necessary conditions unless the burners employed are always 

 identical, and the atmospheric pressure constant. The latter is constant 

 when burnt at a constant barometric pressure ; any alteration in the tem- 

 perature of the surrounding air apparently not altering the relative 

 intensities of the component radiations. Coal gas and candle light appear 

 to be too yellow to use as a standard for white light, unless they be 

 deprived of some of their lower radiations. It has been found that the 

 ' crater ' of the positive pole of the magneto-electric light emits from its 

 central zone a light which is excessively white, and very constant in its 

 component radiations (within limits), the size of the carbon and of the 

 generator being immaterial. At present, testing the light from various 

 specimens of carbons is being undertaken, and not till these experiments are 

 more advanced can any definite idea be given as to whether this source of 

 illumination may be taken as a possible standard. The whole question is 

 so involved in difficulties, instrumental and optical, that it will require a 

 longer period to propose a standard for adoption than it was at first 

 presumed it would do. It would be well, in the face of these difficulties, 

 to enlarge the Committee, so that more workers may be brought to 

 expend their energies on it. 



' This Report was )iot received until after the Annual Meeting, having been de- 

 layed by accident. 



