Page:Proceedings of the Royal Society of London Vol 69.djvu/60

52 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.

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,