198 SEC. 7. LIGHT. 



1st arrangement. Eor objective, the reseau cm smoked glass is used, and 

 for the eye-piece, a common eye-piece. Looking at a jet of gas (for in- 

 stance) at seven metres distance, the deviation of the objective and the eye- 

 piece being from 34 to 41 centimetres (pull one of the tubes), the reflection 

 of the jet is seen reversed, and coloured more or less. By shortening the 

 glass as much as possible, the second reflection is seen green-coloured. 



2nd arrangement. A common objective is used, and for eye-piece, the small 

 photographic reseau. The deviation from the objective to the eye-piece 

 being of 50 centimetres (maximum width of the telescope), the reflection of 

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

 the glass to 31 centimetres, the direct reflection is got as in the Galileo tele- 

 scope. (See Memoire sur les Phenomenes de Diffraction produits par les 

 Reseaux circulates, Archives des Sciences physiques et uaturelles, 1875, 

 vol. 52, p. 320.) 



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

 termining the refractive indices, and the dispersion of any kind 

 of liquids. Carl Zciss, Jena. 



The refractometer enables one to determine the refraction index of a liquid 

 with a single drop of the substance up to four decimals. The indications 

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



917. Procentum Refractometer, 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. 



917a. Refractometer by M. Jamin. 



Polytechnic School, Paris. 



917b. Jamin' s Interferential Apparatus with two 

 Spars. M. Lutz. 



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

 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 Poivell. 



918. Apparatus for demonstrating the Refraction of 



Light in liquids, according to J. Miiller. 



J. Wilhelm Albert, Frankfort~on- Maine. 



The semicircular plate of the refraction apparatus is of glass, ground 

 on its outside and having the scale burnt into it in black. The ray refracted 

 from the liquid appears therefore on the outside of the graduated plate and 

 can thus be viewed by larger audiences. 



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

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



