174 Transactions of the Society. 



polarizing ray, the analyser not being used, has become apparent. 

 The polarizer is also useful in another manner. Eesearches have 

 been undertaken to examine into the influence of the polarizing 

 ray upon substances which may or may not give the usual 

 phenomena under the analyser. Polarized light carefully manipu- 

 lated is very useful in examining thin sections of corals which 

 are made up of closely placed fusiform and long alternating 

 prisms, with geometrical prisms of carbonate of lime in planes one 

 over the other, and often radiating from different points. Shadow 

 and high light succeed when the Nicol is rotated, and minute 

 details become apparent which are not seen, or are only feebly 

 defined, by ordinary light reduced in its intensity to that of the 

 polarizing ray by the use of diaphragms. 



Some time since, in investigating the structure of a fossil which 

 was composed of close radiating and occasionally inosculating tubes 

 with very thin walls and a distinct lumen, all mineralized with 

 calcite in the glassy, non-crystalline form commonly seen in fossils 

 where there is much space unoccupied by structure, the polarizing 

 ray certainly made the tubes more distinct than the ray reflected 

 from the mirror alone, and by rotating the substage Nicol, the 

 position of certain tubes which were invisible before could be 

 ascertained; that is to say, dark lines appeared limiting tubes 

 which were invisible under ordinary illumination. 



The application of the whole polarizing apparatus is very useful 

 in working at the minute superficial structures made up of thin and 

 highly refractive plates of organic carbonate of lime. The glare of 

 light under ordinary illumination and even when the polarizer 

 only is used, prevents the true surface being focussed, or if it is 

 fortunately hit upon, it is more or less invisible. But the analyser 

 being placed across the direction of the polarizing ray the true 

 surface can be found by the definition and distinctness of the clear 

 colours and the intermediate lines. Take away the analyser, and 

 often new structures appear to the eye. As a matter of practice I 

 find that this method is exceedingly useful. 



Many a possessor of a valuable instrument has been discouraged 

 at the outset of his work by the want of definition of his high 

 powers and by the presence of glare in his field, and even of a spot 

 where nothing is visible. He has everything at his command in 

 the way of the instrument, or nearly so. But he works in a room 

 full of difi'used light, and probably uses no diaphragm or cap to his 

 elaborate condenser. He sufiers from reflection in many directions. 

 One of the simplest troubles of the young worker arises from his 

 not preventing the access of hght upon his object when he is 

 working by transmitted light, and the failure when employing high 

 objectives relates to similar causes and requires similarly simple 

 remedies. 



