16 Transactions of the Society. 



bench, which (without the loss of much time, expense, and trouble) 

 is imperative in photomicrography, and even to a certain extent in 

 visual work. 



The position, then, has for some time been one of compelled 

 rest. About four years ago, however, the author, at the Soiree of 

 the Eoyal Society, first saw the replicas of gratings made by the 

 ingenuity of Mr. Thorp. It may be mentioned that a grating is 

 the name employed for a piece of metal which is ruled with lines 

 so close that they number thousands to the inch. The resulting 

 spectrum is very fine, and although the violet end is character- 

 istically more cramped than is the case with the spectrum formed 

 by a dense glass prism, still it is of sufficient size, if properly 

 manipulated, to fill the substage condenser, even with the use of a 

 i-in. objective. It is, perhaps, needless to state that the spectrum 

 from the grating requires practically the same semicircular arrange- 

 ment of the apparatus as demanded in the employment of a prism 

 of the same dispersion, and hence its use for the purpose in ques- 

 tion is prohibited. Mr. Thorp, however, has discovered a means of 

 coating a metal grating (the one selected for my purpose being 

 1% in. square, with lines about 15,000 to the inch) with a solution 

 he has perfected, and which dries as a solid film. When dry, he is 

 able to strip this film off the metal and mount it on a glass sup- 

 port. These replicas are very perfect, and reflect the prismatic 

 colours with unsurpassable perfection ; but in this state, so far as 

 we have as yet described the process, still require the apparatus to 

 be arranged in the semicircular manner previously mentioned in 

 connection with the use of a prism or a metal grating. But they 

 have this difference : being transparent, they transmit a spectrum 

 with as much perfection as a metal one reflects it. 



It is just this difference which constitutes their applicability 

 for the purpose in question. As they can be easily mounted on 

 any glass support, Mr. Thorp now mounts them on corrective 

 prisms of glass of suitable angle, so that he has, after some little 

 trouble, been able to make me one that transmits the violet, blue- 

 violet, and apple-green colours almost in a straight line with the 

 incident light. This enables one to use them with the ordinary 

 optical bench, and so they can be employed both for visual and 

 photographic purposes without arranging any special form of semi- 

 circular apparatus, which is a point of very great importance. 

 They are now commercial, and not at all expensive, some being 

 less in shillings, I think, than the metal gratings cost in pounds. 



As to the little arrangement I have completed, the apparatus 

 (fig. 1) consists of a specially short and compact mixed jet made by 

 Mr. Beard to the pattern desired, with an addition to enable the 

 microscopist to " turn the lime " by means of a handle shown in 

 the diagram from the eye end of the Microscope as it stands on the 

 base-board, which is a great convenience for two reasons : first, it 



