1894.] Measurement of Colour produced by Contract. 221 



Figs. 1 and 2 are summarised in the annexed table, which shows for 

 ach nature of explosive the advantage in velocity and energy to be 

 gained by correspondingly lengthening the gun. 



Fig. 3 is an interesting illustration of a point to which I have 

 elsewhere adverted. Cordite and ballistite leave no deposit in the 

 bore. Bound 1 with R.L.G. was fired with a clean bore. The differ- 

 ence in velocity between round I with a clean bore and rounds 2 and 

 3 with powder deposit in the chase is very clearly marked, and it 

 will be noted that in this instance the effect of the foul bore is only 

 distinctly shown when the length exceeds 40 calibres. 



From 40 calibres onwards the loss of velocity due to a bore 

 encrusted with deposit is very distinctly shown. 



II. "Measurement of Colour produced by Contrast." By- 

 Captain W. DE W. ABNEY. C.B., D.C.L., F.R.S. Received 

 June 5, 1894. 



No definite measurements, as far as I am aware, have been made 

 of the change in colour produced by contrast, except in a small work 

 of my own in which results were given in terms of colour mixtures, 

 and earlier by a brief reference in a work by Rood, in which the 

 change produced was endeavoured to be matched by means of 

 rotating disks. 



The method of registering any colour in terms of some definite 

 wave-length of light, together with white light (see ' Proceedings 

 Royal Society,' vol. 49) renders the registration of any colour 

 readily effected, and by applying it to the contrast colours, very fair 

 results have been obtained, which cannot be very far from the truth. 

 It is usually stated that the contrast colour produced on a white 

 surface by an adjacent colour is the complementary colour, of course 

 largely diluted with white light. I should like to point out that in 

 the first place we have to know what a complementary colour is, 

 and in the second what the added white light may be. As a matter 

 of fact the kind of white light employed has to be defined before it can 

 be stated what the complementary to any colour may be. If, for instance, 

 we wish to define what the complementary of orange may be , we must 

 know what is the na,ture of the white light before we can give the com- 

 plementary. Suppose we take the white of daylight, or of the electric 

 light, we know that to make a white of this character we must add a 

 certain quantity of blue of a certain wave length to the orange. When 

 it is produced under these circumstances, the blue is the comple- 

 mentary to the orange. Suppose, however, we wish to know the 

 complementary to the orange, in what is called the white light of the 

 amyl acetate lamp, or of a candle, we are at once met by a difficulty. 



