1877.] 



Magnifying -power of the Half -prism. 



37 



will be seen how the breadth of the pencil decreases, the rays being thrown 

 to one side, as it were, by the refraction of the prisms. The prisms turn 

 about centres a, h, c, so placed that the rays for different parts of the 

 spectrum, after emerging from one prism, always fall on the same part of 

 the face of the next prism, each being turned through the same angle. 

 There is no difficulty in calculating the positions of these points in any 

 particular case from the condition that = $x as ^ e P r i sm i s turned, and 

 that the lateral displacement of the middle ray in one prism must be equal 

 and opposite to the lateral motion of the perpendicular face of the next 

 prism. For the sake of convenience in mounting (the centres being in the 

 glass itself) the second and third prisms have been made larger than is abso- 

 lutely necessary ; and besides this a large portion of the crown might be cut 

 off as far as optical requirements are concerned. The prisms are each 

 turned through the same angle by the lever d ef, having its fulcrum at 

 / and moved by the micrometer-screw m. 



This lever carries the three adjustable screws p, q, r, so placed as to 

 give the same angular motion to each prism, which is kept in contact with 

 the corresponding screw by a spring on the opposite side. Thus a simple 

 motion of the lever by means of the micrometer-screw brings different 

 parts of the spectrum into the field ; and this automatic motion will not 

 be deranged in the least by withdrawing either one or two of the prisms, 

 which are carried in short adapter tubes to admit of easy removal. The 

 slit and eyepiece can readily be interchanged if great purity in the spec- 

 trum is desired ; and, as just intimated, either one, two, or three half- 

 prisms can be used, giving a dispersion or purity (as the case may be) of 

 four, fifteen, or sixty ordinary prisms of 60°. The concave lens I has the 

 effect of increasing the focal length of the collimator, and consequently of 

 reducing the apparent breadth of the spectrum-lines for a given width of 

 slit. At the same time the spectrum is made narrower and brighter ; 

 if, however, the spectroscope be applied to a large telescope, there is in 

 fact no gain in light, a portion only of the pencil from the object-glass 

 being used, whilst the width and height of the slit may be increased in 

 the same proportion so as to keep the purity and breadth of spectrum 

 constant. Notwithstanding this there is a practical advantage in the 

 diminished effect of dust or irregularities on the slit. It remains to 

 mention one important point in the direct-vision spectroscopes. Since the 

 collimator and telescope are in a straight line, an image of the slit is 

 formed in the centre of the field by rays which pass through the object- 

 glasses above and below the prisms ; and this serves as a bright reference 

 line, which has the great advantage of being affected in the same way as 

 the spectrum-lines by any shift of the collimator or vie wing-telescope. 

 Either a positive or negative eyepiece may be used with this ghost, and 

 its brightness or colour may be varied by diaphragms or coloured glasses. 

 Thus a convenient bright-line micrometer is formed, supplying a want 

 which has long been felt by spectroscopists. 



