428 Transactions of the Society. 



The Microscope has a small convex speculum with a hole in it, 

 close to the back of which the object is placed ; the rays pass from 

 the object through the hole in the convex speculum, and, falling 

 upon a concave speculum, are reflected on to the convex speculum, 

 which reflects them back again through a hole in the centre of the 

 concave speculum to the eye-piece. In brief, it is, as Dr. Smith 

 points out, a Cassegrain telescope (invented 1672), with the object 

 placed close to the back of the convex speculum. 



The history of the catadioptric Microscope is as follows : — A 

 simple form with one concave speculum was proposed by Sir Isaac 

 Newton, but not made. Smith's is really the first, because " The 

 complete System of Optics " was published in 1738, and No. IV. 

 Microscope was made some years before the other three described 

 in that work were computed. Its date may, therefore, be as early 

 as 1730. Barker's catadioptric Microscope (1736) was a Gregorian 

 telescope adjusted to view near objects. A Gregorian used as a 

 Microscope in this manner is merely a toy — a megalascope, or 

 magnifying glass, nothing more, because the object must be placed 

 a considerable distance behind the concave mirror. In order to 

 turn a Gregorian into a Microscope, it would be necessary to drill 

 a hole through the centre of the concave speculum and place the 

 object just at the back of the hole. The aberrations, which in this 

 form are additive, would be large, and the image very bad. The 

 Newtonian method has practical difficulties : (1) If the object is 

 inside the tube, there is the difficulty of illuminating it. C. Tulley 

 showed how this could be done for opaque objects, and W. Tulley 

 for transparent ones.* A Newtonian supplied with both these 

 devices might do very well. Professor Amici's plan was to use a 

 Newtonian telescope with its flat, merely reversing the positions of 

 the eye and the object.f The object was placed outside the tube 

 at the point where the image in a Newtonian telescope is formed, 

 the eye-piece being placed in the mouth of the much lengthened 

 tube. At first sight, no objection would be perceived in this 

 method ; but, if it is examined more closely, it will be noticed 

 that the flat in an astronomical telescope obscures only a small 

 part of the speculum, where the ratio of aperture to focus is about 

 1 to 6, but as the ratio would have to be 1 to 3 for microscopical 

 purposes, the flat would require to be so large that it would cover 

 up too much of the speculum. (2) The objection to the Newtonian 

 Microscope is that the aberration of the single concave mirror 

 must be neutralized by departing from a spherical curve. Dr. 

 Smith's, therefore, is the only real practical solution for a cata- 

 dioptric Microscope : the aberration of one mirror balances that of 

 the other. The chief defect is the absence of any means of 

 illuminating an opaque object ; this, however, might be remedied. 



But to return to Dr. Smith's four Microscopes, it does not seem 



* Micrograpkia. Goring and Pritchard, 1837, pi. 1, figs. 26, 27. 

 t Tom. cit., fig. 28. 



