380 HOW TO SEE WITH THE MICROSCOPE. 



Among the investigators of our own day, 1 will refer you to 

 one whose untiring industry has earned for him a world-wide 

 reputation. I allude to Dr. Lionel S. Beale, F. R. S. Dr. 

 Beale, as you are aware, has been contemporary with, and the 

 supporter of, Dr. Carpenter's heretofore universally received 

 opinions as to the relative capacity of wide and low angled 

 objectives. Dr. Beale, in his work, " How to Work with the 

 Microscope," I quote from the third edition, page 7 says as 

 follows : " For ordinary work it will be found inconvenient if 

 the object-glass when in focus comes too close to the object. 

 This is a defect in glasses having a high angle of aperture. * * 

 Olasses with a high angle of aperture admit much light, and 

 detine many structures of an exceedingly delicate nature, which 

 look confused when examined by ordinary powers. For general 

 work I recommend glasses with an angle of not more than 50Q 

 to 100 Q 



tk Mr. Ross has lately made glasses having an angle of 170, 

 which are valuable for investigations upon very delicate and 

 thin structures, such as the diatomaceae ; but such powers are 

 not well adapted for ordinary work. The importance of 

 arranging the object very carefully, and the necessity of paying 

 great attention to the illumination, render these glasses incon- 

 venient for general observation. The penetrating power of 

 glasses with a low angle is much greater than in those of a high 

 angle of aperture, so that exact focussing is much more impor- 

 tant in the latter than in the former." 



This, in its popular acceptation, is the square doctrine first 

 advanced by Dr. Carpenter, and it has suffered no loss by its 

 filtration through the mind of Dr. Beale. It is the doctrine 

 which has been generally received and accepted. It is the 

 dogma which ten years ago I thought ought not to be true, and 

 of wnich to-night I stand before you the prince of sceptics ! 



In the quotation which 1 have presented, there are two domi- 

 nant ideas ; the one displays the inconvenience attending the use 

 of wide-angled glasses, and to this objection I have already 

 paid my respects in due form ; the other presents the theory of 

 ** penetration " as originally advanced by Dr. Carpenter, sup- 



