Dr. Goring o/i th^ new Microscopes, ^, 111 



together with that of the eye-pieces, the vision may be con- 

 sidered, (barring the circumstance of viewing an image, instead 

 of its real prototype) as mathematically correct, which that of 

 a refractor never can be, for spherical aberration can be per- 

 fectly corrected by the concave of the aplanatics only, to 

 small angles of aperture, scarce sufficient for practical pur- 

 poses. The superior natural brilliancy of single achromatics, 

 with any given angle of aperture, though not absolutely requisite 

 to render objects evident, yet cannot fail to strike the most care- 

 less observer, rendering the picture on the retina much more 

 pleasing and satisfactory : this is another point of superiority 

 which I willingly concede to the refractors. Again, though the 

 catadioptric microscope is more perfect in its principle, and 

 susceptible also of more perfect execution than the achromatics, 

 yet to the refractors must be conceded all those advantages 

 which are included under the head of conveniences and accom^ 

 modations for observations in working tools. It must, I think, 

 be quite needless to insist on this point. How are we to view 

 an object in the bottom of a cavity with the Amician micro- 

 scope, which, perhaps, has not more than 0.1 of an inch of 

 distance between the tube and the object to be viewed ? How 

 can we examine objects contained in a jar of water or other 

 fluid at a considerable depth below the surface ? How are 

 we to find room for managing our tools if we mount it with 

 an erecting eyepiece for dissection, &c. There is still another 

 point which 1 had nearly forgotten, the image of the reflector 

 being many degrees darker than that of a refractor, cceteris 

 paribus, it must follow that there will be a power at which 

 it will cease to be visible, from mere faintness and dulness, 

 when the superior light of the refractor will still be enabled 

 to affect the retina ; and this will take place at last under the 

 most vivid artificial illumination which can be procured. 



1 however deliver it as my opinion that such extremities 

 will never occur with any useful working poiver ; an Amician 

 is capable of showing an opaque object completely well, with 

 a power equal to that of a single lens o^ -^^ inch focus; (which 

 1 can state from my own experience, to be enough for any 

 opaque object I ever met with) on transparent bodies, it will 

 go up to -^ or even ^. 



