52 Dr. E. S. Clay. On the Application oj 



collimator and camera bodies and at the surface of the double-image 

 prism. We reduced this as far as possible by the liberal use of stops, 

 but could not entirely avoid it. This is not wonderful when we 

 consider the small luminosity of the ends of the spectrum, and 

 remember that we were often only using 2 or 3 per cent, of this feeble 

 light. It is obvious that a very small amount of reflected light would 

 be easily noticed. On some occasions the patches were observed 

 through coloured gelatines to help to cut this off. 



For these adjustments I am greatly indebted to my friend, Mr. Alex. 

 A. Tallent, who worked most enthusiastically in getting the apparatus 

 together, and who has assisted me throughout in the experiments. 



The relative intensit}^ of the direct and reflected beams was not 

 constant through the spectrum, and it was necessary to measure it. 

 For as the light strikes the second refracting prism at an angle very 

 near the polarising angle, it is thus partly polarised when it reaches 

 the double-image prism, and therefore the two beams into which this 

 prism splits it will not be of equal intensity, and this inequality is 

 more marked for some colours than for others. 



March 27. I measured a yellow and red ink by Messrs. Mander, of 

 which I had made rubbings. Probably due to small deposits of 

 moisture which form on the prisms and lens owing apparently to the 

 glass being more than usually hydroscopic the inequality above 

 mentioned varies from day to day. The moisture probably reduces 

 the polarisation by reflection at the surface of the prism, and thus 

 alters the relative luminosity of the beams. It is not advisable to 

 clean the surfaces very often for fear of spoiling the adjustments, 

 It is therefore necessary to frequently re-determine the relative 

 brightness of the patches.* 



We found a great improvement in the accuracy of our readings as 

 we became more experienced, and also as the stray light was more 

 perfectly eliminated. The chief difficulty remaining was caused by 

 the unequal density of the inks over the patch, which is far more 

 obvious when measuring them in a monochromatic light than it is 

 when casually observed in ordinary light. 



18. Description of the Curves. 



With this apparatus I measured the proportions of the light of each 

 colour throughout the spectrum that was reflected by several of the 

 inks sold commercially for three-colour printing. 



I measured such a set of inks sent by Messrs. Fleming and Co., and 

 described as " Theoretical Inks " for three-colour work. They are not 

 permanent, but that would not be important for book illustration, 



* The apparatus was entirely re-adjusted after^o. 2 was.measured. 



