366 PHYSICS. 



that they appear to diverge from a point corresponding to the new focal 

 distance of the eyes provided with the spectacles. (Nature^ October, 

 1881, xxiv, C18.) 



Gariel has described a lens of variable focus contrived by Dr. Cusco, 

 for illustrating accommodation in the eye by a variation of the curvature 

 of the crystalline lens. A metallic drum has its ends closed by plates of 

 glass uniform in thickness. A rubber tube communicates with the in- 

 terior at one end and has an elastic bag at the other. The whole being 

 filled with water, jiositive or negative pressure produces at will a convex 

 or a concave lens. {J. Phys., February, 1881, x, 76.) 



Orova has suggested the use of a pair of lenses one i)lano-concave and 

 the other plano-convex, of the same focus, placed in the path of the rays 

 aud separable from each other by a rack-work, for the purpose of varying 

 the magnitude of an image on the screen, when the distance between 

 this and the lantern is fixed. {J. Fhys., April, 1881, x, 158.) 



Pickering, in a paper read before the American Academy, has sug- 

 gested the mounting of a large telescope horizontally, at right angles to 

 the meridian, with a plane reflector inclined 45° to its axis, in front of 

 it. He discusses the possibility of this arrangement, and points out the 

 large number of advantages it would have in sweeping for new objects, in 

 obtaining measures of position, in spectroscopy, and in photometry. 

 {Proc. Am. Acad., April, 1881, p. 364.) 



3. Dispersion and color. 



ThoUon has investigated mathematically the passage of light through 

 a prism, and deduces from his equations the proposition that for every 

 prism there is an angle of minimum resolving i)ower. Further examina- 

 tion shows that for a certain incidence there will be a minimum of reso- 

 lution, i. e., an incidence at which the lines are least well defined, and that 

 at another incidence there will be a minimum of dispersion; these two 

 incidences being symmetrically related to the angle of incidence corre- 

 sponding to minimum deviation. A means of verifying these conclu- 

 sions experimentally is given. {Nature, February, 1881, xxiii, 397.) 



Lippich has examined the question whether it is more advantageous 

 to increase the dispersion or to increase the magnifying power of the 

 telescopes of a spectroscope. He concludes that it is better to increase 

 the dispersion only when the number of prisms does not exceed four or 

 five. His spectroscope of two flint prisms, the light passing twice 

 through them, with a telescope magnifying from 50 to 70 times, excels 

 another instrument having 28 flint prisms, with a telescope magnifying 

 10 times. (Ar/i. J. Sci., November, 1881, III, xxii, 397.) 



Mendenhall has determined the coefQcient of expansion of one of 

 Rutherfurd's speculum metal gratings by means of spectrum measure- 

 ments. The grating was ruled with 8648 lines to the inch, and the wave- 

 length of the line measiu-ed was 5913, an iron line. The range of tern- 



