410 Mr. W. Huggins and Dr. W. A. Miller on the 



tersection of the red and the violet pencils from the last prism ; 

 consequently it falls within the last face of the prism nearest the 

 small telescope. All the pencils therefore which emerge from 

 the prism are, by the motion of the telescope, caused to fall 

 nearly centrically upon its object-glass. The micrometer-screw 

 has 50 threads to an inch ; and each revolution is read to the 

 hundredth part, by the divisions engraved upon the head. This 

 gives a scale of about 1800 parts to the interval between the 

 lines A and H of the solar spectrum. During the whole of the 

 observations the same part of the screw has been used ; and the 

 measures being relative, the inequalities, if any, in the thread 

 of this part of the screw do not affect the accuracy of the results. 

 The eye-lens for reading the divisions of the micrometer- screw is 

 shown at s. 



The mirror /receives the light to be compared with that of the 

 star-spectrum, and reflects it upon the prism e, in front of the 

 slit d. This light was usually obtained from the induction-spark 

 taken between electrodes of different metals, fragments or wires 

 of which were held by a pair of small forceps attached to the 

 insulating ebonite clamp r. Upon a moveable stand in the ob- 

 servatory was placed the induction-coil, already described by one 

 of us*, in the secondary circuit of which was inserted a Leyden 

 jar having 140 square inches of tinfoil upon each of its surfaces. 

 The exciting battery, which, for the convenience of being always 

 available, consisted of four cells of Smee's construction, with 

 plates 6 inches by 3, was placed without the observatory. Wires, 

 in connexion with this and the coil, were so arranged that the 

 observer could make and break contact at pleasure without 

 removing his eye from the small telescope. This was the more 

 important since, by tilting the mirror /, it is possible, within 

 narrow limits, to alter the position of the spectrum of the metal 

 relatively to that of the star. An arrangement is thus obtained 

 which enables the observer to be assured of the perfect corre- 

 spondence in relative position in the instrument of the stellar 

 spectrum and the spectrum to be compared with it. 



5. The satisfactory performance of this apparatus is proved by 

 the very considerable dispersion and admirably sharp definition 

 of the known lines in the spectra of the sun and metallic vapours. 

 When it is directed to the sun, the line D is sufficiently divided 

 to permit the line within it, marked in KirchhofFs map as coin- 

 cident with nickel, to be seen. The close groups of the metallic 

 spectra are also well resolved. 



When this improved apparatus was directed to the stars, a 

 large number of fine lines was observed, in addition to those that 

 had been previously seen. In the spectra of all the brighter 

 * Philosophical Transactions, 186*4, p. 141. 



