242 ADJOURNED DISCUSSION IN LONDON 



plane; as a general rule I fancy it is rather lower down, but the 

 position does not seem to be constant, even in objectives of the same 

 type. If the obliquity of illumination at the margin of the field 

 mentioned by Professor Conrady is to be avoided, the iris will have 

 to be in the back focal plane of the condenser; if that is the case, 

 no lateral movement of the condenser will affect the position of the 

 image of the iris-aperture in the back lens of the objective, and it 

 will be impossible to judge of the centering by looking down the tube. 



Again, I think that both opticians and users of the microscope 

 are content with too little in connection with the performance of 

 the condenser; and I should say that the objection that the slide is 

 composed of " window glass " introduces another " bogey/' The 

 area involved is always small, and if an oil immersion condenser is 

 used, the surfaces of the slip cease to exist optically. At any rate, 

 with a first rate modern achromatic condenser, such as the Watson 

 " Parachromatic," it is possible, when the light source has a screen 

 extending half w^ay across it, to focus with such sharpness an image 

 of the edge of this screen on the object that one row of dots on, 

 say, Pleurosigma Angulatum shall be in full light and the next in 

 " full darkness *' — and this with an N.A. in use of not less than 

 0.7. This means that it is possible to get sharpness of the order of 

 ^ — of an inch. But this is only done on one condition, and 



50,000 -^ ^ 



that is that the distance of the light-source is carefully adjusted to 

 the thickness of the slip; as carefully as we adjust tube-length to 

 the thickness of the cover glass. This point is almost universally 

 avoided by the text-books, and I want to bring it forward as strongly 

 as possible. 



Mr. T. Smith: With regard to increasing the resolving power 

 of microscope objectives, there is little doubt that the numerical 

 aperture, as it is ordinarily understood, can hardly be increased 

 with advantage, buti there is considerable prospect of obtaining in- 

 creased resolving power by using shorter wave-lengths of light. There 

 are very considerable difficulties at present in the way, but I see no 

 reason why they should not be overcome, although an extraordinary 

 amount of experimental work will be involved. It is necessary to 

 know the properties for such light of a very great variety of materials. 

 Where we already possess some knowledge of the behaviour of certain 

 materials with regard to ultra-violet light, this information must 

 become much more precise than at present before it can be con- 

 sidered adequate, and I should like to see some definite encourage- 

 ment given to researches of this character, because they can hardly 

 fail to lead to results of value to the microscope user. Coming now 

 to objectives and their design, it seems to me that this subject has 

 never been investigated systematically, but that new objectives have 

 generally been a further development of old designs on known lines. 

 I should like to see systematic investigations undertaken, so that we 

 may know what prospect there is of effecting real improvements in 

 the corrections. For example, in a high power objective we have a 

 lot of lenses placed very close together, though I am not aware of 

 any thorough investigation which justifies adherence to this arrange- 

 ment. There are obvious difficulties in the way of large separations; 

 nevertheless, there would appear to be some decided advantages to 



