1900] MICROSCOPICAL JOURNAL. 28J 



pecially when it is found necessary to economize the light. 



By far the best source of illumination is a small arc 

 lamp, either hand fed or automatic. When a Naehet ver- 

 tical illuminator is used, the filament of an incandescent 

 electric lamp placed in front of the slit will often give 

 sufficient light. 



If electricity is not available, either incandescent gas, 

 or even a paraffin lamp may be used, but the time of ex- 

 posure will be much longer. Where long exposures are 

 necessary, it is imperative to have the apparatus fitted so 

 as to be quite free from vibration. When using a small 

 arc lamp, the exposures with Lumiere's plates, sensitive to 

 yellow and greeu, vary from two to five seconds; with a 

 paraffin lamp under similar conditions, it would probably 

 be necessary to give an exposure of at least twenty min- 

 utes. Faults in the construction of the apparatus, which 

 are hardly noticed when the exposure is short, become very 

 formidable with a long exposure. 



The camera may be either vertical or horizontal ; for 

 general purposes the latter is much more convenient, and 

 even when using immersion objectives, very little incon- 

 venience will be experienced. It is as well to use a long 

 camera — about seven feet is a very serviceable length — 

 and to have the microscope fitted with a low-power pro- 

 jection eyepiece, the results obtained being invariably bet- 

 ter than when a high-power eyepiece has been employed. 



The most useful magnifications are the 60, 200, 1,000 

 and 2,000 diameters. When using a seven-foot camera at 

 full length, and a low-power Zeiss projecting eye-piece, 

 these magnifications may be obtained with the Zeiss 35 

 mm. projecting, the 24 mm. the 4 mm., with correcting col- 

 lar, and the 2 mm. immersion obiectives respectively. The 

 projecting lens is, of course, used without any eyepiece. 

 Steel for raicrographic purposes is viewed as if it were a 

 rock with various minerals distributed through it, and min- 

 eralogical names are wisely adopted for the constituents. 



