VOL. XXXIX.3 PHILOSOPHICAL TRANSACTIONS. 73 



selves can pretend to. Now instinct must be coeval with the creation, or at 

 least the fail ; and therefore [canine] madness cannot be much younger. 



A Continuation of an Account of an Essay totvards a Natural History of Caro- 

 lina and the Bahama Islands. By Mark Catesby, F. R. S. with some Extracts 

 out of the 8th Set. By Dr. Mortimer, Seer. R. S. N°441, p. 251. 



This part contains some account of the different kinds of snakes and vipers 

 found in those parts. 



A Catoptric Microscope. By Robert Barker, M. D., F.R.S. N°442, p. 259. 



Though microscopes, composed of refracting glasses only, have been vastly 

 improved, as to their effects of magnifying ; yet they have been attended with 

 such great inconveniences, that their application to many arts, in which they 

 might be very convenient, is not so common as might be expected, and man- 

 kind have reaped but a small part of the advantage obtainable from so surprising 

 and useful an instrument. 



Among the inconveniences mentioned, the following are the most con- 

 siderable : 



1. That in order to magnify greatly, it is necessary that the object-glass be a 

 portion of a very minute sphere, whose focus being very short, the object must 

 be brought exceedingly near; it will therefore be shaded by the microscope, and 

 not visible by any other light than what passes through itself; in this case 

 therefore, opaque objects will not be seen at all. 



2. Objects, illuminated this way, may be rather said to eclipse the light, than 

 to be truly seen, little more being exactly represented to the eye, than the out- 

 line ; the depressions and elevations within the out-line appearing like so many 

 lights and shades, according to their different degrees of thickness or transpar- 

 ency ; though the contrary happens in ordinary vision, in which the lights and 

 shades are produced by the different exposure of the surface of the body to the 

 incident light. 



3 . Small parts of large objects cannot easily be applied to the microscope, 

 without being divided from their wholes, which in the case of vivi section de- 

 feats the experiment, the part dying, and no more motion being observed 

 in it. • " •'*' 



4. The focus in the dioptric microscope being so very short, is exceedingly 

 nice, the least deviation from it rendering vision turbid ; therefore only a very 

 small part of an irregular object can be seen distinctly this way. 



To remedy these defects. Dr. Barker has contrived a microscope on the 



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