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the object all the light entering the base of the paraboloid. No. 1 lens was 

 calculated for nse with a slide of ordinary thickness, say a sixteenth of an 

 inch. No. 2 lens was calculated for use with a slide not exceeding one- 

 hundredth of an inch in thickness, and he had constructed " drum-head " 

 mountings for objects between thin glass, which enabled an object to be 

 viewed and lighted indifferently on both sides. Parallel light should be 

 thrown into the base of the paraboloid, r and the most splendid effects were 

 obtained by means of direct sunlight. The object might safely be illumi- 

 nated up to apparent incandescence, without danger to the objectives. By 

 a set of central stops, peripheral zones, and revolving shutters, the light 

 might be narrowed horizontally, and thrown into the object at any angle 

 and in any azimuth. One curious fact had come out, that the valves, now 

 shown upon the table and so finely resolved under the light of this new 

 paraboloid were not resolvable by Mr. Lealand with his former methods. 

 This was owing to the fact that the valves being set upon the slide, there 

 was a stratum of air between them and the cover which reduced the correc- 

 tion of the lens so much that it lost that extension of angle by which it had 

 power to resolve an acus valve when attached to the under surface of the 

 cover — the space between the object and the lens being then wholly occupied 

 by glass-cover plus water, instead of as at present partly by air. This fact 

 raised the question how much the value of " angle of aperture," depended 

 simply upon the reception of additional light from the object, how much 

 upon angle qua angle. But this he would not now attempt to discuss. It 

 was, however, important to note that the pure brilliant positive image of an 

 object, as given by this paraboloid, was resolvable by a lens with smaller 

 angle of aperture than was another image, and, if we could by new 

 means of illumination increase the resolving power of low angled lenses, a 

 vast gain would be made. The defining and resolving power of an astrono- 

 mical telescope depend directly upon the area of objective — other things 

 being equal. Here the difference consists purely in bringing into the image 

 a larger number of pencils of light from the planets or other objects, the 

 question of angle qua angle not coming into account. The telescopic image 

 is a positive one, and the blacker the field the finer the impression made 

 upon the retina. In the microscope, the negative image only being seen by 

 transmitted light, the most luminous area before the eye was necessarily the 

 blank field, while the object was recognized only by its power of obstructing 

 light, and was comparatively weak in its impression upon the retina. With 

 this positive image, on the other hand, there was a soft black background, 

 and the brilliant image of Podura or Acus as thus shown, obviously had 

 immense advantages in regard to its power to impress itself upon the retina. 

 With bacterial fluids the effect was equally remarkable. Saliva, blood, &c, 

 viewed by a good dry quarter of about 95°, were seen almost as new ob- 

 jects, when lighted up by this paraboloid. 



Mr. Ingpen said that the Club was highly indebted to Dr. Edmunds for 

 bringing the matter before it, and the subject was certainly one of very 

 great interest. It was also a curious instance of the way in which an in- 

 vention might be made before the time came for its due appreciation, and 



