Measuring- Apparatus for a Laboratory- Spectroscope. 109 



Reading off Positions of Spectral Lines. — In commencing an 

 observation it is always desirable to see that the point of the 

 index A stands at the zero of the glass scale when the telescope 

 is in the corresponding position on its scale. Any adjustment 

 of the index that may be necessary is easily made in the way 

 already described — namely, by firmly holding the rod r and 

 turning the milled head which carries the index to the desired 

 extent. The actual reading of the position of a line to which 

 the point of the fine needle in the eye-piece is brought is then 

 made from the glass scale. 



An exceedingly feeble light suffices to enable the operator 

 to read the wide divisions on the white scale ; but in observing 

 very faint lines I do not read by reflected light, but faintly 

 illuminate the scale by means of a very small gas-jet or lamp 

 placed behind it. Sufficient light is transmitted by the opal 

 glass to enable the readings to be easily and quickly made, 

 while the eye of the operator is retained in a sensitive 

 condition for feeble rays. Moreover, in reading, it is not 

 necessary to move the head away from the eyepiece of the 

 instrument. 



I have tried with success a mode of determining small dif- 

 ferences with this apparatus, which could doubtless be applied 

 with advantage in mapping spectra with instruments of high 

 dispersive power. 



The glass plate B was removed from its stand and the index 

 from the rod r ; I then attached to the latter a cork carrying 

 a small mirror placed at a suitable angle. A spot of light was 

 reflected from this mirror and made to fall on a screen placed 

 several metres away. The relative distances between the 

 members of groups of closely ruled lines (those of the nitrogen 

 spectrum) were then easily determined in this manner, as the 

 actual motion of the needle from point to point was greatly 

 magnified. 



The relative positions and widths of the lines seen with the 

 instrument are easily laid down on a millimetric scale. I have 

 had a number of 200-m.m. scales printed on narrow slips of 

 paper ; and the graduations are lithographed on a band of six 

 equidistant lines, which thus serve for marking off intensities, 

 according to Bunsen's graphic method. One millimetre cor- 

 responds to one unit of the scale on the opal-glass screen, and 

 consequently to three minutes as read off with the vernier on 

 the graduated arc of the instrument. Differences corresponding 

 to 1' can therefore be easily estimated and represented on the 

 millimetric scale. 



But one other practical point need be mentioned. I find it 

 exceedingly convenient to mark off on the opal-glass scale the 



