SIR HERBERT JACKSON 215 



by ordinary transmitted light, what is seen is a brown or yellow- 

 brown object. I am not going into the theory or the details, because 

 it would take too long, and I am speaking to experts who, I am 

 quite certain, know perhaps better than I, what is the explanation 

 of the brown colour. But how many people who have looked at 

 it simply as an object have asked " Why is it brown? " If you 

 take that same object with a black ground illumination and a 

 low angle objective, using an illuminating cone of .4 numerical 

 aperture, all you see is the outline of the specimen- 

 Yet what a wealth of information is to be gained from 

 an investigation of the inside of that outline; Mr. Barnard 

 has well indicated that when he was speaking of work in 

 connection with the yeast cells. If you raise that cone to .65 

 with a black ground illumination, with the sam<e objective, 

 the object then is a beautiful blue or violet colour. Raise it still 

 higher and it gets nearer to a greenish colour, and if you put on 

 a little higher angl,e lens with an immersion condenser, the object 

 looks very nearly white; raise the angle still a little higher and the 

 image is white. That is an illustration of why every change should 

 be explained and interpreted. Take another illustration: tous-les- 

 mois starch grains mounted in water. With a black ground and an 

 objiective of a numerical aperture of .26, it is really a pretty object. 

 The grains are nearly all pearly white, and the concentric rings can 

 be seen quite well. If you keep the same objective, but raise 

 the numerical aperture of the condenser, all the beautiful light goes, 

 and nothing more than a, mere outlin^e can be seen ; it looks like a 

 little ring of light with nothing inside. Raise the aperture of the 

 objective and use an immersion condenser, and you begin to see 

 a little more showing up inside, and that is the first indication of 

 the existence of a structure there. I should like the same thing to 

 be taken with an opaque illuminator and examined. Opaque illum- 

 ination, except in the examination of metals, has not had the atten- 

 tion paid to it that it should have because we have not laid sufficient 

 stress on the necessity of looking at the object from all points of 

 view, so as to deicide by a careful comparison of the appearances in 

 every possible form of illumination, the correct and proper 

 interpretation. The student might be encouraged to go through a 

 systematic course of theoretical and practical microscopy, applying 

 what he learns in lectures to the study of objects of comparatively 

 well-known structure by their examination with optical systems of 

 increasing power and with various forms of illumination until he has 

 gained a real knowledge of what can be revealed by the microscope 

 and of what are its limitations. With this experience he would be 

 in a position to proceed to research work equipped with sound theory 

 and the fundamental practical knowledge necessary for the inter- 

 pretation of what he sees and the avoidance of hasty judgment 

 through incomplete observations. 



If we could look forward to educational work somewhat on these 

 lines in the future, people who wanted to study microscopy would 

 find there was a great deal of valuable work to be done in extending 

 the use of the instrument. The brilliant work which Mr. Barnard 

 is doing in connection with ultra-violet light and increased resolving 

 power cannot but help us very much in interpreting many things 

 which we have seen but have not understood. 



