SCIENTIFIC NEAVS. 



[July 1st, 1887. 



of lines and dusky bands, and, lastly, in the violet, the lines 

 H and Hi are at the centre of two shaded stripes. The 

 spectrum beyond this is very faint, and of a lavender colour ; 

 it can be made more visible by allowing it to fall on a screen 

 prepared with a solution of quinine, or on a cell containing 

 sesculin, a substance contained in the bark of horse-chestnut. 

 The ultra-violet spectrum, as it is called, is then distinctly 



vernier. The telescope not only magnifies the spectrum, 

 but enables the eye to take in all the light falling on its 

 object-glass, instead of only the small amount which could, 

 be received by the pupil of the eye. 



The prism is usually of flint glass, and has an angle 01 

 60 degrees. In order to ensure that the collimator brings 

 the rays parallel, the prism should be removed, and the 



seen of a soft blue colour, and it is crossed by lines which 

 have been described by the letters L, M, N, O, P, O, R, 

 and S. 



It is necessary for the production of a good spectrum 

 with straight lines that the rays of light falling on the prism 

 should be parallel, not divergent. Fraunhofer, who made 

 his observations with a telescope, placed the prism and 

 telescope 24 ft. from the slit. 



Mr. Sims introduced a simple method of obtaining parallel 

 rays much more conveniently by placing a lens between 

 the prism and the slit, and arranging the focus so that the 

 rays falling on the prism are parallel. Such a lens is called 

 a collimator. 



Fig. 5* represents the simplest form of spectroscope, if 

 the small pocket instruments be excepted. A pillar sup- 



ports a graduated circle, in the middle of which is a turn- 

 table carrying the prism. The slit is formed between a 

 pair of knife edges, and its width may be adjusted by a 

 screw. The slit is carried at one end ot a tube, which is 

 furnished with a collimating lens at the end facing the prism. 

 A telescope magnifying eight to ten times, mounted on a 

 moveable arm, can swing round the graduated circle, and 

 its angular position may be determined by the scale and a 



* We are indebted to Mr, J. Browning for the illustrations of Figs. 5, 

 6, 7, and 8, 



Fig 6 



telescope focussed on a distant object ; that is to say 

 focussed for practically parallel rays. It is then turned 

 until it is in a line with the collimator, and this is then 

 adjusted till the slit is seen distinctly. The prism is then 

 replaced and may be set, together with the telescope, to the 

 position of minimum deviation. A third tube is provided 

 in more elaborate instruments, and carries a scale photo- 

 graphed on glass ; the image of this is thrown by reflection 

 from one side of the prism, up the telescope, in which it is 

 seen together with the spectrum. 



More than one prism is employed in most researches. 

 The instrument illustrated in Fig. 6 is used for astronomical 

 observations. It is provided with two prisms and a cylin- 

 drical lens instead of a slit. A star would give a spectrum 

 of no appreciable width, but observed through the cylin- 

 drical lens it gives a bright line, which is observed as if it 

 were the slit of an ordinary spectroscope. 



In some researches very great dispersion is required, and 

 to produce this a train of prisms is used ; the light passing 

 through each in succession is refracted more and more, and 

 at the same time suffers more and more dispersion. It has 

 been remarked that the necessary condition of minimum 

 deviation must be observed in order to obtain a pure spectrum . 

 With a single prism this can be easily arranged by the use of 

 the turn-table, but with a train of prisms, each one has to be 

 turned to the proper angle. Fig. 7 shows a train of prisms 

 provided with a very ingenious means of giving the neces- 

 sary twist to each prism. The prism on the left-hand side 

 of the illustration is fixed to the circular table, and all the 

 others are connected by hinges. A slotted plate attached to 

 each prism is guided by a "central pin which stands up from 

 a bent lever. This lever is attached to the telescope, and 

 the arrangement is such that when the telescope is moved 

 to sweep along the spectrum from one end to the other, the 

 prisms automatically place themselves so that at any 

 moment they are in the position of minimum deviation. 



