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XXXIV. Note on a very Convenient Method of Accurately 

 Focussing and Adjusting the Image of a Laboratory Light 

 Source upon the Slit of a Spectrograph. By Gr. A. 

 Hemsalech *. 



IT is often necessary in exact spectroscopic investigations 

 to project by means of a lens a sharply defined image 

 of the source of light upon the collimator slit, for the purpose 

 of differentiating between the radiations emitted by various 

 regions in the source. In this way Sir Norman Lockyer, to 

 whom this most important method of spectroscopic observa- 

 tion is due, discovered the so-called long and short lines in' 

 the arc and spark. In order to make the requisite adjust- 

 ments it generally suffices, especially in the case of such 

 bright sources as the arc or spark, to observe the image on 

 the slit and to move either the source or the lens until the- 

 image appears quite sharp. But in the case of a feeble 

 source of light, such as an uncoloured flame, or a source 

 which is limited in duration as, for example, a small high 

 temperature plate resistance furnace, the usual method of 

 adjusting the image on the slit becomes difficult and even 

 impracticable. Having become confronted with this incon- 

 venience in the course of my experiments with plate furnaces, 

 I was led to try several alternative methods, and finally 

 adopted one which not only entirely solved the difficulty for 

 the furnace, but was also found to be equally well applicable 

 to the flame, arc, or spark. This method is merely the con- 

 verse of the old one, and consists in focussing the slit upon 

 the source of light. It is really so simple that I have no 

 doubt of it having already been used by others, but since I 

 have not seen a record of it in any work on spectroscopy, 

 and also in view of its unquestionable usefulness, I venture 

 to give a short account of it for the benefit of those spectro- 

 scopists who may not yet be acquainted with it. 



Let us suppose that the projection objective has been 

 centred with respect to the axis of the collimator by means 

 of some suitable method, as, for example, the one I have 

 given in my thesis t, and that the slit S is at one of the foci 

 of 0. If a spark passing between the electrodes E and E' 

 be placed at the conjugate focus of its image will coincide 

 with S. But conversely also an image of the slit S is formed 

 at T coincident with the spark-gap. It is this image T which 

 is observed and made to coincide with the tips of the elec- 

 trodes E and E'. In practice .the procedure adopted is as 



* Communicated by the Author. 



t Hemsalech, Theses de Doctored, Paris, 1901, p. 33. 



