318 KEPOET— 1881. 



The brass mounting in whicli the scale is placed is always so made as 

 to admit of movement horizontally, so that any division of the scale may 

 be adjusted to any given line. The adjustment for the Bunsen scale 

 is made by bringing the sodium line to 50 of the scale, the image of that 

 edge of the slit which does not move when the breadth of the slit is 

 altered being made to coincide exactly with the division 50. If this 

 be on the left hand of the observer, then always the position of the 

 left-hand edge of each line and band is to be observed, and in the case 

 of a faint line the slit may be opened to admit more light, and yet an 

 accurate reading raay be obtained. This refers, of course, only to lines 

 which are sharply defined, and not to bands of considerable breadth. 

 The most convenient plan in making a map of an ordinary spectrum 

 is first to put down, as exactly as possible, the positions of the well-defined 

 lines on an ordinary lithographed millimetre scale, opening and closing 

 the slit as convenient, and then to go over the woi'k again, keeping the 

 slit at one uniform width and noting the relative intensity of the lines 

 and the width and character of the bands, whether sharply defined at the 

 edges, or sharp at the one edge and fading away at the other, or bright 

 in the middle and fading away at each edge. There is no better plan 

 of noting the peculiarities of a spectrum than that employed by Bunsen, 

 in which each bright line is represented by a black mark on the paper, 

 whose height represents the intensity of the line. 



A convenient modification of the scale used with the spectroscope for 

 ordinary purposes has been proposed by Professor Emerson Reynolds.^ 

 The observing telescope cai-ries cross wires, and as it moves from one line 

 to another it causes an index-finger to travel round over a divided arc 

 on a plate of opal glass, which is feebly illuminated bj'^ a small flame. 

 The positions of the more important lines of the elements, whose spectra 

 are easily obtained with the Bunsen flame, are marked on the opal plate ; 

 the identification of any particular element is thus made without moving 

 the head away from the eyepiece of the instrument. 



Very beautiful drav.ings of many of the ordinaiy spectra are given 

 in Lecoq de Boisbaudran's ' Spectres Lumineux.' The means of ignition 

 employed in producing these spectra were (1) the ordinary Bunsen flame, 

 (2) the spark from an induction coil (without a Leyden jar) striking on 

 the surface of the solution of the substance to be examined, (3) the spark 

 impinging on the surface of the fused salt, (4) the spai'k between metallic 

 "wires. In some cases, the gas feeding the Bunsen burner was charged 

 with hydrochloric acid gas, by making the gas pass through a flask 

 containing a warm solution of hydrochloric acid. The spectra drawn 

 comprise the flame-spectra of cEesium, rubidium, and potassium chlorides ; 

 barium chloride, bromide, and iodide : strontium, calcium, magnesium, 

 manganese, copper, and gold chlorides ; salts of sodium, lithium, thallium, 

 and boracic acid ; and the spark-spectra of salts of potassium, sodium, 

 lithium, barium, strontium, calcium, magnesium, aluminium, chromium, 

 manganese, iron, cobalt, nickel, zinc, cadmium, indium, tin, bismuth, 

 lead, antimony, copper, silver, mercury, gold, platinum, and palladium, 

 besides absorption-spectra of chloride of didymium, chloride of erbium, 

 and potassium permanganate. Accurate drawings are given by Bunsen^ 

 of the following spectra : — Flame-spectra of potassium, cassium, rubidium, 



' Phil. Mag. (Fifth Series) vol. v. p. 106. 



- Pogg. Ann. der Physik n. Chemie, civ. 366. Phil. Mag. (Fourth Series) vol. 1. 

 p. 527. 



