14 Transactions. 



the appearances of minute structure under the microscope, 

 should we not always be glad to accept every means in our 

 power to modify and improve our illumination ? It does not 

 follow that because a test is resolved that we have the best re- 

 sults our lens is capable of giving. It certainly appears from 

 the discussion before the Quekett Club, where this new con- 

 denser was exhibited, that the appearance of these very tests 

 was somewhat novel. Another point made by Mr. Edmunds 

 tends to confirm an opinion of long standing. He suggested 

 that " if we could, by new means of illumination, increase the 

 resolving power of low angled lenses, a vast gain would be 

 made," and it seems already that he has done this to a certain 

 extent by this very paraboloid. 



Admitting the value of these accessories as aids to investi- 

 gation, we return to our original question, what is the best 

 kind of illumination ? Taking a difficult frustule of a dia- 

 tom let us observe the effect of various methods with the 

 mirror. With direct light we can see the outline, with oblique 

 light the fine markings, and as less and less light passes 

 through the object in a line with the optical axis, the more dis- 

 tinct become the markings. We might justly conclude from 

 this, that the proper method of illumination would not be 

 by transmitted light. 



In practice we only secure the best resolutions of lines or 

 series of dots, when the line is of the utmost possible obliquity. 

 Now, the ordinary way of looking at objects in every-day life 

 is by reflected light. We are easily deceived as to a transpar- 

 ent object when seen by transmitted light. Naturally this is 

 also'^the case when using the microscope, and I would speak in 

 favor of reflected light for this purpose. In fact, I am 

 strongly inclined to the opinion that the most appropriate and 

 best light for microscopical work is what comes from above 

 the stage. What we want is a positive image of the object, 

 and this cannot be obtained by the ordinary methods of work- 

 ing. One of the greatest obstacles to this kind of illumina- 

 tion has heretofore been the difficulty of throwing sufficient 

 light upon the object with high powers. A yV can now be 

 used very successfully in this way with simply the mirror above 

 the stage. 



Practically, a positive image is also obtained, when the ob- 

 ject is transparent, by means of very oblique light from such 



