164 SUMMARY OF CURRENT RESEARCHES RELATING TO 



ioiv^ Uibe and fine edge, stops all reflections. These Lister's dark-wells 

 were generally supplied in three sizes ; but in later years the tubes were 

 made shallower, and they were not as effective as the old pattern. 

 Having got the dark-well in position, and supposing we wish to examine 

 an opaque foram, we focus down upon it and raise the lieberkiihn until 

 we have the light, which is an image of the bull's-eye, concentrated 

 brightly upon the object. 



Secondly. We wish now to see one of the transparent forms ; but 

 if we look at it with the adjustments as they were used for the opaque 

 foram, the transparent foram will be so dark as hardly to be visible. 

 What, then, must be done ? All that is necessary is to focus down the 

 lieberkiihn, and when the correct position is reached the transparent 

 foram will shine up quite brightly. So we see that with the same slide 

 and same object-glass and lieberkiihn different forms of forams require 

 different foci of the lieberkiihn. These remarks apply equally to poly- 

 cystines and diatoms, as well as to various parts of insect structures, etc. 

 These important facts have never been published anywhere, neither in 

 text-ljooks, nor in any paper read before the societies ; and yet they 

 form the basis for the proper use of a lieberkiihn ! The theoretical 

 explanation is that the spherical aberration of the lieberkiihn mirror 

 enables one to illuminate the opaque object by light which falls more or 

 less directly upon it from above — i.e. light of small angle from the 

 central portion of the lieberkiihn ; but Avhen the shorter focus of the 

 lieberkiihn is employed, the semi-transparent object is illuminated by 

 wide-angled oblique light from the outer zone of the liel)erkiihn. Day- 

 light gives only poor results on objects mounted in a medium. In 

 conclusion, we have to discuss the relative merits of illumination by 

 lieberkiihn and by condenser with stop. Some may think that they 

 are in effect the same, and may therefore be used indiscriminately ; but 

 such, however, is not the case. Let two Microscopes be provided — say 

 Wenham binoculars — as similar as possible, with two 1-in. objectives, 

 one with a lieberkiihn and the other with a condenser and stop ; obtain 

 two similar slides of forams, and select for examination similar specimens. 

 The image with the condenser and stop may be very brilliant, and may 

 take in those who do not understand the way to intei'pret a correct 

 image : but a little thoughtful attention will show that the condensed 

 image has a great deal of the underneath part of the foram confusing 

 the image. of the upper part, which with the lieberkiihn image is not 

 the case. 



Caution. — All lieberkiihns are not properly made or fitted. The 

 objective should have a long, smooth snout, and the lieberkiihn should 

 slide smoothly upon it, and, what is very important, have sufficient 

 range of motion. 



The objectives and lieberkiihns as made by Messrs. R. and J. Beck 

 are excellently arranged. Some lieberkiihns will be found to have such 

 long tubes that they will not focus the light upon an opaque foram, 

 while others are so deeply cupped that they touch the slide before they 

 focus the light. The first fault is easily remedied by reducing the 

 length of the tube, but the second must be corrected by the maker. 



Finally, it can be stated that anyone who has not seen a Eupodiscus 



