668 PROCEEDINGS OF THE QUEKETT MICROSCOPICAL CLUB. 



subtractive relations. In the former case the stage should be 

 rotated till the fast direction of the section is parallel to the slot. 

 It may happen, then, that the section is neutralised by one of 

 the steps, and therefore is of the same relative retardation. If 

 one step just fails to neutralise and the next higher will more 

 than do so, and neither are completely dark, then if they be 

 equally bright, the relative retardation must be midway between ; 

 if one is darker, then it will be proportionately nearer to that 

 step. In this way the relative retardation can be estimated to 

 within 20 or 30 micro-m.m. When the directions-image i.e. 

 the object viewed without an eyepiece reflected on the back 

 lens of the objective is examined between crossed nicols, it 

 shows in the centre of the field the same interference colours as 

 that seen in the object image. The colours move with the stage 

 as it rotates without suffering any changes of configuration, 

 x^t the same time the field is traversed by dark bands or 

 brushes, which constitute the isogyre. As the rotation proceeds, 

 these change both their position and their shape and may from 

 time to time leave the field altogether. When the stage is in 

 the position corresponding to extinction in the object-image or, 

 in other words, when the vibrations in the section are parallel 

 to the cross wires, the isogyre passes through the centre of the 

 field and is known as a ' : central isogyre." 



Dr. Evans concluded his lecture by describing the technical 

 indications of the different isogyres. 



The lecture was fully illustrated by diagrams and coloured 

 lantern slides, which were, Dr. Evans pointed out, mostly due 

 to the art of Mr. C. H. Caffyn. 



There was a collection of photographs of rock sections on 

 Lumiere Autochrome plates, exhibited by Messrs. J. W. Ogilvv 

 and C. H. Caffyn, and also a series of exhibits showing the pro- 

 cess of mounting a rock section. 



