Echelette Grating for the Infra-Red. 773 



metal above the original surface at the edges. The first 

 gratings were ruled on copper and subsequently gold-plated 

 to prevent tarnish, but it was found that even the lightest 

 polishing on the bulling wheel destroyed the sharpness of the 

 edges and caused the development of strong central images. 



It was found, however, that even with an exceedingly thin 

 deposit of gold (about the lightest plating ever done com- 

 mercially) it was possible to rule very deep grooves without 

 uncovering the copper. This solved the difficulty, and 

 excellent gratings could be produced at a very small cost. 

 The copper plate was varnished with asphalt on the back to 

 save gold, and gilded in as large pieces as the gilding 

 establishment could handle. These large sheets v\ ere then 

 cut up to the required size with a circular saw. They per- 

 formed fairly well optically, giving almost as good images as 

 an ordinary plate-glass mirror, in spite of the rather rough 

 treatment to which they had been subjected. It is important 

 to instruct the gilder to do as little burring as possible. My 

 first plates were spoiled by having too thick a deposit of gold 

 and too vigorous buffing or burnishing. The best treatment 

 is the one which they give to the thinnest coats, which would 

 be completely removed if polished by the methods employed 

 for thicker deposits. If a thick deposit is given and polished 

 in the usual way, the optical surface is ruined by the formation 

 of undulations, though it is hard to convince the gilder that 

 it is unsatisfactory. I mention these details for the benefit 

 of others undertaking the manufacture of these gratings, 

 for it look me nearly a week to convince the gilder that 

 he could be taught anything about the nature of metals 

 and how they should be treated. To obtain a better optical 

 surface, or rather a flatter one, I had a polished flat plate of 

 speculum metal, such as is used for making Rowland gratings, 

 sdver-plated and polished. The circumstance that the first 

 plate which I placed in the hands of the plater flew into three 

 pieces as soon as he put it in the hot alkaline solution which 

 they use for cleaning thin metalwork, convinced him that there 

 was something about metals to be learned, and he was more 

 willing to take advice thereafter. By this method it was 

 possible to get a beautiful optical surface of soft metal, in 

 which the grooves could be cut. So far as I could see by a 

 rather superficial examination, the optical perfection of the 

 surface had not been materially affected. 



The angle of the ruling edges of the carborundum hexa- 

 gonal plates is 120°, consequently the sides of the groove 

 make approximately this angle. By placing the crystal in 

 various positions we obtain grooves of various si 



