62 PEACTICAL PHOTO-MICEOGEAPHY 



will better illustrate this point, and it will be seen that the 

 appearances are practically reversed so far as light and shade 

 are concerned. 



The reason of this is fairly obvious ; when the illumina- 

 tion takes place by projecting the light at an angle of 

 approximately 45 to the surface, the beam is reflected and no 

 direct light enters the objective unless some alteration of 

 its direction is caused by the structure or irregularities of 

 the object itself. A smooth polished surface will under these 



6 



FIG. 16. Frosted Silver-leaf Film on Glass ; (a) by Oblique Light, (6) by 

 Normal Light, x 250. 



circumstances appear practically black. If now the same 

 object is illuminated by a vertical illuminator, the conditions 

 are reversed ; the rays are condensed on to the surface of the 

 object by means of the front lens of the objective, and are 

 totally reflected by the polished surface, passing up through 

 the objective again and causing the surface of the object to 

 appear brilliantly illuminated. The specimen selected to 

 illustrate this point is that utilised by Mr. G. T. Beilby in his 

 work on ' The Surface Structure of Solids/ and consists of a 

 frosted silver surface on which a burnisher mark has been 

 made. In both these objects there is a certain amount of 

 light scattered ; but where the burnisher has passed over, the 

 irregularities are almost obliterated, with the result that this 

 mark under oblique light appears black, and with vertical illu- 



