ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 857 



Observation of Opaque or Quasi-opaque Objects in the Micro- 

 scope. — Dr. John Anthony writes us as follows : — 



Given a Microscope, an objective, and a good " bull's-eye " side- 

 light, the examination of opaque or quasi-opaque objects would seem 

 to be a very simple affair, but experience teaches that much manage- 

 ment is required to bring out all the points of structure in an object, or 

 in making such object show at its best in any subsequent examination. 

 It is therefore hoped that the following practical results of many 

 years' work with the Microscope will be received by the Society in 

 the spirit in which they are tendered. 



A very large class of objects examined by aid of the Microscope 

 are either opaque or semi-opaque. No one can doubt that if we are 

 able to make out the structure of an object by merely looking upon 

 its surface, the result is far more satisfactory than can be got by any 

 process of seeing through it. But some objects which are semi-opaque, 

 though the amount of transparency be but small, lend themselves to 

 a combination of both methods, and the effect of this double lighting 

 properly balanced is charming and instructive beyond expression. 

 This may be instanced in the examination of the beautiful whole-insect 

 preparations of Mr. Euock. The mere " bull's-eye " side-light would 

 not reveal half the structure, and no one knows better than Mr. Enock 

 himself the enormous gain by supplementing the light of the bull's- 

 eye by a flood of transmitted light, illuminating in this case the body 

 of the insect by means of the achromatic condenser, and balancing by 

 careful manipulation the respective amounts of light ; the effect, when 

 properly got, is little less than magical, so much so, that even the 

 advanced microscopist would not find his time wasted by practising 

 this double-illumination on some rather intractable object. 



In this double-lighting sometimes a better effect can be got by 

 using the spot-lens instead of the achromatic condenser, in aid of the 

 bull's-eye or silver side-reflector. In my hands the bull's-eye, which 

 is really a French " crossed lens," of some 5 in. diameter, and so giving 

 a flood of light, does not yield nearly so pure an image as a good 

 parabolic reflector, which in its turn is somewhat troublesome to use. 



Where there is a want of transparency in the object, then the use 

 of the Lieberkiihn comes in with advantage. This Lieberkiihn was 

 much in vogue in the early days of the Microscope, and its use is 

 not to be despised for certain objects ; it fell into disfavour from a 

 tendency to illuminate an object equally all round, and to afford no 

 contrast of light and shadow, no " boldness of image " ; but this quasi 

 defect is very readily obviated by blacking about one-third of the 

 silver reflecting surface with Indian ink, which does no sort of 

 damage, and so a preponderance of light on the one side can be got 

 of a very pure cpuilify, and with the advantage — a very great one, 

 that with the finger and thumb grasping the tube of the Lieberkiihn, 

 the illumination can be made to revolve in azimuth, and so bring out 

 salient points under every condition of light and shadow — a mode of 

 verification which no searcher for truth would be disposed to 

 neglect. 



It has been assumed so far, that white light has been used, both 



