APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 163 



with the small- sized Siemens' differential lamp, and also with 

 the Giilcher, which is very steady if a proper resistance coil 

 be interposed in the circuit. Others also doubtless give good 

 results. But the most important point in an optical lamp is 

 the concentration of the radiant into one luminous point. 

 All the usual lamps fail in that particular ; and in perfecting 

 the arrangements of the electric projecting microscope, 

 especially, I was continually baffled by the confusion caused 

 by the two luminous points or poles, which was only partially 

 remedied by setting the positive carbon behind the negative. 

 For some time, pointing this out, I endeavoured in vain to 

 induce various makers to construct an optical lamp with 

 carbons inclined at an angle of 30 or 40, after the manner 

 well known in the hand- regulated ' projector ' lamps often 

 used at sea. 



Being finally advised by Professor S. P. Thompson to 

 adopt for other reasons the Brockie-Pell lamp for microscopic 

 work ; and finding upon inquiry that Mr. Brockie had already 

 constructed tot Mr. Phillips a simpler arc lamp for lantern 

 ?jse, adjusting the arc by the potential of the current, in which 

 the negative carbon worked up by a kind of candle-spring to 

 a fixed stop, so as to ' keep in focus,' while the arc could be 

 readily adjusted to any current within reasonable limits, I 

 urged upon him very strongly to further perfect it for some 

 microscopes then constructing at Messrs. Newton's, by giving 

 the carbons the slant just alluded to. He at once undertook 

 to do this (my sole suggestion in the matter), and the final 

 result of the additional alteration was a lamp whose external 

 appearance can be seen in the representation of the electric 

 microscope on p. 209. This, as an optical radiant, is far the 

 best I am acquainted with ; and up to this date is, I believe, 

 still the only focus-keeping regulator-lamp whose carbons 

 occupy the proper inclined position. It is extremely port- 

 able, steady upon its base, very simple hi mechanism, and 

 its cost is only fifteen pounds. It is not the lamp usually 



