THE MICROSCOPE. 181 



was directed immediately below the object on the slide, thus 

 avoiding all stray rays. A very small piano convex lens cemen- 

 ted to the end of the rod by means of Canada balsam served 

 the purpose of concentrating the entire beam upon the object 

 under examination. The great facility with which this was 

 done showed that the apparatus was equal to, if not superior, 

 to the Abbe condenser system. 



There was sufficient illumination for the manipulation of the 

 objects, and the great comfort obtained by the use of this so- 

 called " dark lantern " more than paid for the cost of operating 

 which was slight. This arrangement rendered the use of the 

 e} T e shade, a device which is often used by the amateur micro- 

 scopist, entirely unnecessary, another point in its favor. 



The total refraction of the rays of light made it possible to 

 have these light-conducting cylindrical glas3 rods of almost any 

 shape. A bend may be made without any difficulty, and there 

 can be little doubt but that they may be used as scale illumina- 

 tors in well constructed electrical instruments. 



The heat arising from the enclosing of the incandescent 

 lamp in the reflector may be done away with by drilling sev- 

 eral holes at the top and bottom of the encasing metal. A 

 draught will thus be produced which will cool the heated inter- 

 ior. This heating is sometimes so intense that in one partic- 

 ular instance, where a green glass incandescent lamp was used 

 in a parabolic reflector, some of the reflected rays were caught, 

 by the green glass and absorbed, and in a comparatively short 

 time the globe was softened, thus causing it to collapse. 



The portion of the glass rod to be soldered to the brass tube 

 should be prepared as follows : Coat it evenly with a dilute 

 solution of platinic chloride, and deposit the metallic platinum 

 by dipping into an essential oil, such as lavender. A bright 

 grayish conduction film is thus obtained, which may be electro- 

 plated in a sulphate of copper bath. If an even and pliable 

 coating is obtained, it may then be treated as an independent 

 close-fitting tube, and may be soldered accordingly. Care should 

 be taken, however, not to heat it too much in the act of soldering, 

 for unequal expansion is liable to cause either a fracture or a 

 loosening of the copper coating. If this should happen the 

 glass rod may be cemented into the tube with some material, 

 such as plaster of Paris, which is not affected by heat. 



