'96 SUMMARY QF CURRENT RESEARCHES RELATING TO 



case, the carbon poles themselves, either one or both, being the source 

 of light. 



It is. with this lamp, quite easy to obtain monochromatic light, as it 

 is obviously only necessary to screen off the bright lines in the spectrum 

 which are not required, and the one which remains will then constitute a 

 source of light which is not merely monochromatic, but is of one wave- 

 length. The brightest lines in its spectrum lie in the region of the 

 orange-yellow, green, and blue-violet, and it is these three that are <>f 

 use. There are a number of faint lines, but for the purpose now 

 described they are not of any importance, and are not sufficiently bright 

 to interfere in practice with the result. The necessary colour-screens 

 can be made by staining gelatin films with a suitable dye, or a more 

 exact and convenient method is to use glass cells in which is placed a 

 solution of the dye employed. By means of a direct- vision spectroscope 

 it is easy to observe the exact concentration of the solution that is 

 required, and no undue absorption of light therefore occurs. 



The following combinations of dyes in aqueous solution have been 

 found satisfactory : — Eosin and filter yellow K (Fuerst Bros.) will 

 filter out all but the orange-yellow line. The eosin should be 

 sufficiently concentrated to exclude the green line, the filter yellow K, 

 being used only to subdue the violet and ultra-violet. This screen is 

 perhaps the one of most value for either visual or photographic work, as 

 the position of the line in the spectrum is that of the greatest visual 

 luminosity. In photomicrography its application will be sufficiently 

 obvious. Naphthol-green and filter yellow K will give a light that is 

 visually a brilliant green, but spectroscopically transmits some yellow 

 as well. The green, however, predominates so largely that for visual 

 work it is very useful where a considerable quantity of light is required. 



Tartrazine will transmit the yellow and green lines, but in this case 

 the yellow predominates, the green being somewhat subdued. To 

 obtain the green line only, a solution of acid-green must be used 

 together with filter yellow K, and this gives a source of green light for 

 microscopic work, either visual or photographic, which it is difficult to 

 imagine can be improved upon. The violet line is more easy to isolate, 

 as it can be filtered off with a screen of methyl-violet or gentian-violet. 

 It lies rather far in the spectrum towards the ultra-violet, so that visually 

 it is not of great use, but its possibilities in photography are obvious. 



The illustration herewith shows the form of lamp made by the 

 Bastian Co., and suitable for microscopic work. It has an automatic 

 tilting device, so that immediately the current is switched on the arc is 

 struck and the lamp lights. The process is therefore similar to the 

 starting of a carbon arc, in which the two poles have to touch one 

 another before any current passes or light is produced. 



When the mercury bridges over the gap between the poles and is 

 allowed to flow back again, some mercury is vaporised in the tube and 

 the light is at once emitted. 



The length of the glass tube is dependent on the voltage of the 

 supply, and the polarity of the current must be arranged so that the 

 mercury commences to vaporise at the negative pole, the residual 

 mercury being driven back into the bull) at the positive pole. 



For microscopic work it possesses the additional advantage that there 

 is practically no radiant heat. 



