II. SELECTORS. 217 



971c. Curve for obtaining Wave-lengths of Spectra; and 



Map of the absorption spectra of bromine and iodine monochloride. 



Professors Roscoe and Thorpe. 



857a. Automatic Motion for the Spectroscope. 



Walter Baily. 



The apparatus consists of an axle and four parallel discs, the outer pair being 

 fixed, and the inner pair rigidly connected together, and capable of turning on 

 the axle. Between the inner discs are four arms, which also turn on the 

 axle. 



Taking the centre of the axle as origin, each disc has 4 slits, the equation 



to their middle lines being r = F(0), r = ^(*\ r = F (?) r = F (*)- The discs 



in each pair are placed with their slits parallel, but the inner discs are turned 

 over. The arms have straight slits radiating from the axle. A pin is passed 

 through the first slits of the outer discs, the 4th of the inner discs, and the 

 slit in one of the arms. The remaining arms are connected with the discs by 

 three other pins inserted in a similar manner. The first and last prisms are 

 fixed on the initial lines of the outer and inner pair of discs respectively, and 

 the four other pi-isms are carried by the arms. Motion is given by moving 

 the inner discs. The angles at which the slits .cross are constant and differ 



f\ / g~ 



least from right angles if F(0) = e~~ / , which is the form adopted in the 

 model. 



859a. Photo-Spectra of Metals and Gases, obtained 

 with a Browning Direct-vision Spectroscope. 



John Rand Capron. 



These spectra were photographed with wet plates by J. R. Capron and 

 G. H. Murray, for use and reference in connection with auroral investigations. 



The optical apparatus consisted of a Browning direct-vision prism, with, in 

 the case of metals, a collirnating lens of 6-inch focus, and a projecting lens 

 of 9-inch focus. For the gases a similar prism was used, with collirnating 

 and projecting lenses, each of 4-inch focus. The photographs are enlarged 

 to twice the originals. The electrical apparatus consisted of a 4^-inch spark 

 Ruhmkorff coil, worked by four half-gallon bichromate cells, a condenser of 

 four glass-coated plates, and spark terminals. 



The spark was placed about half an inch from the slit. Mr. Lockyer's 

 plpn of interposing a lens was tried in some cases, but given up, as the long 

 and short lines were found to be equally well observed in the spark itself. 

 The gases were mostly in Geissler tubes. It is proposed in further experi- 

 ments to get rid of the air lines by taking the spark between the metal elec- 

 trodes in glass bulbs, through which a gas passes. 



One of the spectra represents the spark between platinum electrodes taken 

 in a current of coal gas passing through a tube. 



The dispersion of the direct-vision prism is as follows: A =17 '62; 

 B = 23'62; C = 30'42; D = 50; E = 77'35; F=103'90; G=159'73. With 

 the instrument as used for the gases, it is found possible to photograph 

 faint spectra satisfactorily. 



b. POLARISCOPES, &C. 



861. Jellett's Saccharometer, for the measurement of the 

 rotation which certain fluids are capable of producing in the plane 

 of polarisation of the transmitted ray. Trinity College, Dublin. 



