1834.] On Catadioptric Microscopes. 291 



the former was not sufficient to reach these faint objects, the latter 

 showed them perfectly well." 



But to return to our humble sphere ; the advantages of the new con- 

 struction may be very briefly summed up. First, increased brillian- 

 cy and penetrating 1 power, arising from the removal of the plane. 

 Secondly, the probability of obtaining finer instruments; for the artist 

 having the difficulties of but a single reflecting surface to contend with, 

 manifestly enjoys at least a double chance of producing a perfect woi-k. 



Of the method of observing with this instrument, I think it hardly 

 necessary to speak. The object must be inserted into the tube, affixed 

 either upon a very small plate of talc, or upon the point of a fine needle ; 

 and adjusted to the focus with a delicate screw. This is a very simple 

 affair in the hands of a skilful mechanic. For observing large objects 

 with low powers, a speculum of long focus, say 1^ or 2 inches, is 

 obviously the best ; but for more minute investigations a deeper one is 

 desirable. With the requisite arrangements, such an instrument would, 

 I conceive, be hardly if at all more difficult to use than a refractor. 



I have wrought several metals upon this plan, and though I enjoy no 

 opportunity of having them fitted up otherwise than in the rudest and 

 most imperfect manner, I find their performance upon some minute and 

 rather difficult objects very satisfactory. With a metal of about six inches 

 focus and three aperture, without any attempt at adjustment, or any con- 

 venience for arranging the focus, several test objects have been easily 

 developed, such as the asperities on the surface of the human hair, the 

 striae upon the dust of lepidlopterous tribes, &c. Fig. 5. represents the 

 hair of the mouse, like apiece of well twisted whipcord, with longitudinal 

 marks between the spirals, considered by microscopists as a good test. 

 (Dr. Goring in the Quart. Journal, June 1827,) A represents the hair 

 near the root, B near the extremity. 



When I first applied myself to these constructions, I was apprehensive 

 that the introduction of light would produce a glare in the tube detri- 

 mental to vision. But I find this not at all the case. The main tube 

 however, should be made considerably larger than the diameter of the 

 mirror, and a well blackened diaphragm may be placed on each side the 

 illuminating apparatus to quench any straggling light that may intrude. 

 The size of the diagonal plane in the above instrument is about .05, in 

 shorter diameter, and is I find a great deal too large. One of only .025 

 would be sufficient, I think. I had almost omitted to mention, that as 

 many objects require a somewhat oblique radiance to be even seen, this 

 may be obtained by giving the diagonal a slight revolution on its shorter 

 axis ; or perhaps as simply by closing up one half of the bull's-eye 

 lens. 



p p 2 



