MICROSCOPES. 201 



should have a microscope of simple construction. It is 

 far better to have one of the simplest form with very few 

 movable parts, than to have a microscope with all the 

 movements 1 have described badly executed. If you want a 

 cheap microscope, have a simple one, but do not get a cheap 

 microscope which has a great many movements which work 

 irregularly, give tremor, or throw the object out of focus. 



I now come to the practical use of the instrument. Assuming 

 that you have a microscope of a satisfactory kind, when you 

 come to use it success depends as much on properly under- 

 standing how to illuminate the object as on the quality of the 

 instrument itself. You may have a splendid microscope, but 

 unless you illuminate the object properly, you will see nothing, 

 for the simple reason that there is nothing to see. I think I 

 could not do better on the present occasion than deal some- 

 what at length with this part of my subject, since it is of 

 such great importance in the practical use of the microscope. 

 First of all then, I will draw your attention to the surface 

 illumination to which we must have recourse in examining 

 opaque objects, and which may sometimes be used with ad- 

 vantage in the case of objects which are not opaque. With 

 this kind of illumination very much depends on light and 

 shade, as in the cases of objects which we see with the naked 

 eye. We all very well know that if you have light thrown 

 directly on a wall it may appear to have a perfectly uniform 

 surface ; but if the light be thrown very obliquely all the 

 irregularities of the plaster, blisters and such things, are made 

 very conspicuous. This illustration serves to show that if 

 you have the light thrown at a certain angle you can see no 

 character whatever ; but if thrown at another angle the surface 

 structure may be seen as well as could be desired. In some 

 cases if you were to throw the light at too oblique an angle 

 certain structures might be hid by the shadows of other 

 greater irregularities, so that for each particular object it is 

 very important that you should throw the light at particular 

 angles, and in examining an unknown object it is desirable to 

 try the effect of light of various angles of obliquity. There 

 are various ways of obtaining this surface illumination. We 

 can have a bull's-eye condenser throwing the light at various 

 angles from the side, or we can use a silvered parabolic reflector, 

 the first of which was made for my own use in examining iron 

 and steel. One advantage of these parabolic reflectors is that 



