208 LECTURES TO SCIENCE TEACHERS. 



light and the diameter of the sub-stage opening, than can be 

 actually seen with any one single illumination. What I am 

 anxious to do is by an inductive process to be able by further 

 examinations to form something like an intelligible ex- 

 planation of the cause of the lights and shadows that we see 

 in objects as examined by transmitted light, and in order to 

 do this it is most important to be able to vary the illumina- 

 tion very much, and at the same time to know exactly what 

 its true character is whether slightly or very divergent, or 

 proceeding from a small or wide sub-stage opening. 



Independently of objects whose structure is shown by lights 

 and shadows in the way I have alluded to, we have many 

 which are seen by the difference in their colour, arid then it 

 is only a question of illuminating the surface, or transmitting 

 light through the object. In sections of rocky substances, 

 the different constituents, black, red, brown, or otherwise, are 

 seen at once by their colour, independently of any light or 

 shade. This method of distinguishing objects by difference 

 in colour is extensively employed in studying organic struc- 

 tures, by using a staining on which they act like mordanted 

 textile fabrics. Various staining materials will thus combine 

 with certain constituents and colour them, and have a very 

 slight effect on others. In that manner you recognise the 

 different constituents of what otherwise would appear like 

 almost homogeneous structure. 



Another important question connected with the microscope 

 is illumination by means of polarised light. I must take it 

 for granted that you all know what polarised light is, since 

 time would not permit me to describe its characters. One 

 principal point is, that polarised light has different properties 

 in different directions. We can sometimes make use of it 

 very effectively independent of an analyser, by having a 

 polariser under the stage of the microscope and illuminating 

 the object with polarised instead of with ordinary light. 

 Polarised light, when the plane of polarisation is inclined in 

 certain directions to an edge of a crystal or other inclined 

 surface, may be to a great extent reflected and not transmitted 

 to the eye in cases where the amount reflected would be very 

 much smaller if you used ordinary light. In this manner 

 you may define crystals or other transparent objects mounted 

 in Canada balsam, which is of so nearly the same refractive 

 power that they can scarcely be seen with ordinary light. 



