1 62 OPTICAL PROJECTION 



action all the time with one nozzle or the other. In many 

 optical experiments also it is required to push the radiant to 

 a considerable distance from the condenser, and this also 

 necessitates that the cylindrical body should be fairly large. 



Of recent years a great many new patterns of science 

 lanterns have been placed on the market by various firms, 

 and it is now possible to obtain an instrument suitable for 

 nearly all optical experiments, as compact, and nearly as cheap, 

 as those made for slide work only. 



Space here forbids further mention of these smaller science 

 lanterns ; but a more detailed account of the various forms 

 will be found in the appendix at the end of the book. 



84. Arc Lamps. The Duboscq lamp is too familiar to 

 need description. It is only fit for battery use, and for inter- 

 mittent experiments, being incapable of giving a really steady 

 light. An equally serious fault is the length of the are 

 between the carbons, the centre of the radiant being dark, 

 between two luminous poles. With about forty Bunsen or 

 Grove cells, however, this lamp answers the purpose for 

 ordinary optical experiments, and to get an even disc with it 

 is possible, though it requires more skill than most assistants 

 possess. Mr. Cottrell, formerly demonstrator at the Royal 

 Institution, stood almost alone in his power of getting a 

 fairly steady and well-centred light with this form of regu- 

 lator. 



The lamp known as the Suisse-Serrin is a very much 

 better lamp of this class. It uses larger carbons, and I have 

 known it, in the hands of my friend the Rev. P. R. Sleeman, 

 give a nearly steady light for a continuous half-hour. For 

 battery use only, this is an excellent lamp, and by setting the 

 positive carbon somewhat behind the other, the evils of a 

 double radiant can be partially avoided. 



For the dynamo currents of modern days, however, this 

 lamp is not well adapted, and many regulators now in the 

 market give better results. I have seen very good work done 



