238 Transactions of the 



limited, and of course all structures that could not be seen in balsam, 

 sucli as many butterfly scales, were quite excluded. I may remark 

 also that immersion object-glasses (which, however, were not known 

 or employed at that time) were not available, as the water between 

 the cover and lens would prevent the reflexion, and the object could 

 not be seen opaquely, I was not then aware of the possibility of 

 viewing objects in a dark field with the largest apertures when 

 mounted dri/. This was an accidental discovery made a few years 

 afterwards. Happening by mistake to take up my Podura slide 

 and place it on the immersion paralx)la, I was surprised to perceive 

 a few of the very numerous scales that I knew were there, shining 

 out with great brilliancy, and found that these were the ones in 

 contact with the slide itself; all those on the cover were invisible. 

 It thus became evident that the efi'ect was due to the interruption 

 of total reflexion from the slide, the mere contact or presence of 

 the object thereon attracting the light, or allowing it a passage 

 through. That the object should under these circumstances collect 

 light enough to be seen under high magnifying powers, could not 

 have been anticipated ; but so great is the intensity, that Col. Dr. 

 Woodward has obtained some fine photographs of objects illumined 

 this way. Except to friends at hand, I gave no special description 

 of this method. The first notification of it appeared in one of my 

 discussional communications to the ' Monthly Microscopical Journal ' 

 for July 1, 1869. Being usually either too indolent or indifferent 

 to make public demonstrations in order to disseminate my notions, I 

 merely publish them and leave them to be appreciated by the public 

 or not according to their merits. Perceiving that many of our most 

 eminent observers now use the truncated lens and parabola for this 

 curious principle of illumination, I have exerted myself to improve 

 upon the original plan, now sixteen years old, and which I must 

 confess is in many respects a most clumsy and unhandy combina- 

 tion, and so difficult to manipulate, that experienced microscopists 

 sometimes fail to show satisfactory results. In the first place it is 

 difficult to patch on the truncated lens in the exact spot under the 

 object you wish to observe. You have to shift it about several 

 times before you can do this. After much trouble having got 

 everything right for one object, you wish to have a look at a neigh- 

 bouring one, you have to traverse the slide — lens and all — which 

 will thus be set excentrically both in the parabola and relative to 

 the object, and then to get a passable view you are compelled to 

 effect an imperfect compromise by shifting either the mirror, the 

 outstanding bull's-eye lens (which is always required for parallel 

 rays), or by working the parabola up and down, and worst of aU, 

 if particles of dirt or diatoms are thickly strewn over the slide, 

 they all become so brilliantly luminous, that the whole field is flooded 

 with blue and white blurs, which greatly injure the definition by 



