BOCKETT LAMP. — GAS-LAMP. 



141 



Fig. 85. 



denser, which is thus raised or lowered with the lamp itself, far 

 more conveniently than when mounted on a separate foot. The 

 flat wick may be so turned as to present to the mirror or condenser 

 either its whole breadth, or only its 

 edge, or any intermediate aspect ; 

 the light in the second case being 

 much increased in intensity, but re- 

 stricted to a smaller surface. — To 

 every one who has a supply of Gas at 

 command, the use of it for his Micro- 

 scope-lamp (by means of a flexible 

 tube) strongly recommends itself, on 

 account of its extreme convenience, 

 and its freedom from any kind of 

 trouble. The lamp should be con- 

 structed on the general plan already 

 described, the burner being made to 

 slide up and down on a stem rising 

 perpendicularly from a foot, which 

 also carries a shade ; and the burner 

 should be one which affords a bright 

 and steady cylindrical flame, either 

 ' Leslie's' or the 'cone' burner being 

 probably the best. Even the best light 

 supplied by a Gras-lamp, however, is 

 inferior in quality to that of a good 

 Oil-lamp ; and is more injurious and 

 unpleasant to the eye. Hence the 

 interposition of some kind of artificial medium adapted to keep 

 back the yellow rays, whose predominance in the lamp-flame is 

 the chief source of its injurious action, is especially required when 

 Graslight is used. This may be partly effected by the simple expe- 

 dient of using a chimney of bluish glass, known as ' Leblond's ; ' 

 but, in addition, it is advantageous to cause the light to pass 

 through a screen of bluish-black or neutral-tint glass ; and it will 

 then be nearly purified as to quality, though much reduced in 

 intensity.* Mr. Eainey, who has paid great attention to the best 

 means of obtaining a good illumination by artificial light, recom- 

 mends, as the best moderator, one piece of dark-blue glass free 

 from any tint of red, another of very pale-blue with a slight shade 

 of green, and two of thick white plate-glass, all cemented together 

 with Canada balsam ; this, as already stated, may be used with 

 Sunlight, as well as with Lamplight. 



106. Position of the Light. — When the Microscope is used by 

 Daylight, it will usually be found most convenient to place it in 



* A Gas-lamp provided with these and other appurtenances for 

 regulating the illumination, and also with a water-bath and mounting- 

 plate, has been devised by Mr. S. Highley. 



Bockett Lamp. 



