290 On Catadioptric Mlscroscopes. [June, 



that almost any amount of loss may be compensated by an artificial con- 

 densation of light upon the object under examination. But the disad- 

 vantages of a double reflection are of a higher and more important or- 

 der. It is one of the most inflexible laws of practical optics, that all 

 superfluous refractions or reflections are to be avoided, and for this sim- 

 ple reason, that to form a perfect surface, either plane or spherical, or 

 of any other figure, is beyond the power of human art, however exqui- 

 site, and hence the greater the number of reflections or refractions, the 

 more darkened and muddy will the vision become. When we consider 

 therefore, how extremely minute are many objects of microscopic inves- 

 tigation (such for instance as the marks, probably not the 10,000th of 

 an inch in diameter, upon the dust of lepidlopterous insects), we maycon- 

 ceive how much vision may be impaired by a very slight error in the 

 figure of the plane, augmented in the long passage of the rays from the 

 mirror to the eye-lens. It is well known that Newton's objections to 

 the Cassigramian and Gregorian telescopes arose from considerations of 

 this kind. " The errors (says he) of the said convex will be much aug- 

 mented by the too great distance through which the rays reflected from 

 it must pass before their arrival at the eye-glass. For which reason I 

 find it convenient to make the tube no wider than is necessary, that 

 the eye-glass be placed as wear to the oval planes as possible, &c." If 

 we conceive his own form of the instrument subjected to reversed vision, 

 it will bear no remote resemblance to the Amician microscope; and 

 tried in this way, I am pretty confident we should find a material differ- 

 ence in the performance of the most exquisite reflector. I took a one- 

 foot Newtonian, having a very indifferent plane, but which showed 

 objects in the day time sufficiently well ; and making the rays from the 

 object enter by the small metal, I looked with a leas directly in front, 

 throwing the image a little out of the axis so as to escape the plane. 

 Allowing for the necessary deficiency of light, the image was so con- 

 fused and distorted, that it was almost impossible to distinguish any 

 object whatever. 



As there is some analogy between the telescopic " front view" of 

 Herschell (magnis componere parva !) and the microscope I propose, 

 it may not be amiss to remind the reader of the advantage that great 

 optician found in laying aside the oval plane. Besides the " capital 

 advantage of nearly double the light of former constructions," the de- 

 fining power seems also to be increased. " The 20ft. reflector having 

 been changed from the Newtonian form to my present one, I had a 

 very striking instance of the great advantage of the increased pene- 

 trating power in the discovery of the Georgian satellites. The improve- 

 ment, by laying aside the small mirror, was as 61 to 75, and whereas 



