322 



The curves of the figui'e illustrating this report are dra-wn from the 

 same data. 



The dififerent methods of measuring the positions of the lines of a 

 spectrum may conveniently be put into two groups, which may be called 

 methods of consecutive coincidences, and methods of simultaneous coinci- 

 dences. The chief plans employed are the following : — 



' Consecutive Coincidences.' 



(1) The graduated arc and vernier. 



(2) The tangent-screw micrometer. 



(3) The bright line micrometer. 



' Simultaneous Coincidences.' 



(4) The reflected scale. 



(5) The double- wire micrometer eyepiece. 



(6) The divided-lens micrometer. 



(7) The photographic method. 



It is not necessary to remark that some methods are more suitable for 

 a small spectroscope, and others for a large one, and again, that a par- 

 ticular method may be employed in one case and not in another ; for 

 example, cross- wires can be employed with the solar spectrum or with any 

 spectrum of sufficient brightness, while they are useless with very faint 

 spectra. 



A favourite plan with the opticians is that of the divided arc and 

 vernier, in which the telescope candies cross-wires, the intersection of 

 which is brought to coincidence first with one line, then with a second, 

 and so on. This of course is a method of ' consecutive coincidences,' and 

 it is a necessary condition of obtaining correct results that the collimator 

 and slit shall remain rigidly in the same position and that the cross-wires 

 of the telescope and the vernier shall retain the same relative position 

 during the motion from one line to another. These conditions are attended 

 to in the massive construction adopted by Steinheil and some other con- 

 tinental makers, but are fatally disregarded when the instrument is 

 constructed of slender metal, and when the collimator and obser-ving 

 telescope, instead of being firmly grasped at the centre of gravity, are 

 merely screwed by one end into a slender upright of brass, further 

 weakened at the most important point by being attenuated into some 

 (so-called) ornamental shape. Certain precautions must be observed in 

 the use of a spectroscope with cross-wires to obtain good results. The 

 eyepiece should first be removed and so adjusted that on looking through 

 it at a sheet of white paper, the cross-wires are seen in sharp focus, then 

 replacing the eyepiece in the observing telescope removed from the 

 spectroscope, the telescope should be exactly focussed on a distant object. 

 Having replaced the telescope in the instrument, the collimator should 



