230 SEC. 7. LIGHT. 



2nd arrangement. A common objective is used, and for eyepiece the small 

 photographic grating. The distance from the objective to the eyepiece 

 being of 50 centimetres (maximum length of the telescope), the image of the 

 gas jet, reversed, is obtained as in an astronomical glass. By pushing in the 

 tube to 31 centimetres, the direct image' is got as in the Galileo telescope. 

 (See Memoire sur les Phenomenes de Diffraction produits par les Reseaux 

 circulates, Archives des Sciences physiques et naturelles, 1875, vol. 52, p. 

 320.) 



915f. Diffraction Grating on speculum metal. A. Hilger. 



916. Refractometer, according to Abbe's system, for de- 

 termining the refractive indices, and the dispersion of any kind 

 of liquids. Carl Zeiss, Jena. 



The refractometer enables the determination of the refractive index of a 

 liquid to be effected up to four decimals with a single drop of the substance. 

 The readings refer to line D, and are read off from a graduated sector. 



917. Procentum Refract ometer, for determining the per- 

 centage of solutions and mixtures by optical means. 



Carl Zeiss, Jena. 



The instrument is designed for liquids -whose index lies between 1 3 

 and 1 4. The determination takes place at a numbered scale in the field of 

 view of a small telescope. Besides the scale for the absolute index of re- 

 fraction, there is another scale, which gives directly the per-centage strength 

 of saccharine liquor. 



91 7a. Refractometer by M. Jamin. 



Polytechnic School, Paris. 



917b. Jamin's Interference Apparatus with two 

 Spars. M. Lutz. 



91 7c. Refraction Goniometer, constructed by the Rev. 

 Baden Powell, and used in some of his experiments, and after- 

 wards by the Rev. T. Pelham Dale and Dr. Gladstone in their 

 earlier researches on refractive indices of liquids at different tem- 

 peratures. Mrs. Baden Powell. 



919. Abbe's Refractometer, for determining the power of 

 refraction of different liquids as far as the fifth decimal, with direct 

 reading of the refractive index, without calculation. (Comp. 

 Abbe : " Neue Apparate.") 



Franz Schmidt and Haensch, Berlin. 



921. Apparatus on J. Miiller's Principle, for Experi- 

 ments on the Refraction of Rays of Light in Fluids. 



Warmbrunn, Quilitz, and Co., Berlin. 



92 la. Apparatus by M. Mascart for studying the Re- 

 fraction of Gas. 



M. Mascart, Professor at the College of France. 



