8 Transactions of the Society. 



tight. When using a small arc lamp, the exposures with Lumiere's 

 plates, sensitive to yellow and green, 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 minutes. 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 inconvenience will be experienced. 

 It is as well to use a long camera — about seven feet is a very service- 

 able length — and to have the Microscope fitted with a low-power pro- 

 jection eye-piece, the results obtained being invariably better than 

 when a high-power eye-piece had been employed. 



The most useful magnifications are the 60, 200, 1000, and 2000 

 diameters. AVhen 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 collar, and the 2 mm. immersion objectives re- 

 spectively. The projecting lens is, of course, used without an eye- 

 piece. 



Steel for micrographic purposes is viewed as if it were a rock 

 with various minerals distributed through it, and mineralogical names 

 are conveniently adopted for the constituents. 



Pure iron, being an elementary body, is made up of one substance 

 only, to which the name " ferrite " has been given. A sample of 

 ferrite, magnified 850 diameters, is shown in fig. 4. It will be seen 

 that it is composed of a number of interlocking crystals. 



Steel is composed of iron containing approximately from ' 05 to 

 2 • per cent, of carbon, and it has the peculiar property of becoming 

 much harder when it is made red-hot and quenched. The carbon in 

 steel which has been slowly cooled is combined with a portion of the 

 iron, forming iron carbide, which is known as " cementite." This 

 constituent contains about 7 ■ per cent, of carbon, and remains bright 

 after a polished section of the steel is attacked by an infusion of 



EXPLANATION OF PLATE I. 



All fiss. X 850 diameters. 



Fig. 3.— Iron and eutectic (iron the plane, and eutectic the laminar structure). 

 „ 4. — Pure iron (ferrite). 

 „ 5. — Steel containing - 6 p.c. of carbon (pearlite the dark, and ferrite the whit* 



constituent). 

 „ C. — Steel containing 1*8 p.c. of carbon (pearlite the dark, and cementite the 



white constituent). 

 „ 7. — White cast iron containing 2 - 2 p.c. of caibon (pearlite the dark, and 



cementite the white constituent). 

 „ 8. — Mottled cast iron containing 3S p.c. of carbon (pcailite, cementite, and 



graphite). 



