672 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of the dye to a teaspoonful of water ; warm until dissolved ; filter if 

 solution contains any foreign matter, and add it to about an ounce and 

 a half of the stock solution of gelatin whilst still warm, and therefore 

 in a liquid condition. Now clean some f in. circular cover-glasses and 

 place them on a sheet of white paper. With a glass rod deposit a little 

 of the warm gelatin solution on the centre of one of them, and quickly 

 lower upon it another cover-glass, pressing it down to remove super- 

 fluous liquid. The gelatin will set almost immediately, with the result 

 that a thin film of it, protected on both sides from injury, will be 

 obtained. Such films can be cleaned like an ordinary cover-glass with 

 no fear of their coming apart. Some background stops will now be 

 required, and these can be prepared as follows. With a fine camel-hair 

 pencil paint a circular disc of the coloured gelatin solution in the centre 

 of a cover-glass, and allow it to dry. Care should be taken to put it 

 on thinly and evenly, and a neater job will be made of it if a fine 

 ring of varnish be first put on the cover with a turntable, afterwards 

 painting in the central area. Two or three dozen films should be pre- 

 pared whilst the materials and solutions are about, as they are always 

 handy. 



Early Glass Micrometers. — Mr. E. M. Nelson presented at the 

 June Meeting two micrometers for the cabinet of the Society, and has 

 supplied the following description. These two micrometers are interest- 

 ing as being early specimens of a glass micrometer. They are ruled 

 on the slides (2 by f in.), and no cover-glass is used. They are both 

 ruled in squares, one in y^, and the other in ^^ of an inch. I have 

 compared the -^jj with an accurate micrometer with some care, and find 

 that the average of the ^hy i s slightly in excess, viz. T -|~g-. The greatest 

 interval is y-^-, and the least o^g 5 probably an error of two units in 

 the fourth decimal place was not thought much of in those early days 

 of micrometry ; one of the interspaces however is only TTT V q-q in. in 

 excess. The r ^ was only cursorily examined, but the ruling seemed 

 more even ; but the error, like the other, was in excess of the truth. 



Probably these micrometers were ruled by Powell, as they belonged 

 to a Microscope made by him in 1838. 



Method of Demonstrating Newton's Colours by Transmitted 

 Light.* — It is well known that, if white light be passed through a thin 

 film, part of it will be reflected twice within the film and will cause 

 interference and colour phenomena. These are usually very faint be- 

 cause the amount of light which is thus reflected is so small as compared 

 with what passes directly through, -as to have but a slight effect. If, 

 however, the same wave-front be passed through a uniform series of 

 films, successive portions of certain colours should be blotted out in 

 each film, while other colours which get through the first film without 

 interference, should emerge from each of the other (similar films) with- 

 out interference, and the colour effect should be cumulative. At the 

 suggestion of Prof. Barus, these surmises have been empirically verified 

 and excellent results obtained. If a number of wire ring's of the same 



& fc 



Amer. Journ. Sci., xv. (1903) pp. 224-5. 



