40 Dr Goring on Reflecting Microscopes. 



concave to a perfect hyperbolic figure is extreme, and, when 

 combined with a fcrfect polish, taxes human skill to the very 

 uttermost ; — the parabola in its curve or contour comes nearer 

 to the sphere than the hyperbola, it is therefore more easily 

 hit ; let any workman try which will be figured first, an hy- 

 perbolic metal of 3 inches aperture, and 6 inches focus, or a 

 parabolic one of the same diameter and 9 inclies focus, yet both 

 these metals will make a telescope of equal dumpiness, (suppos- 

 ing the Gregorian tohave its small metal of 1 inch focus), for the 

 metals will be at much the same distance apart. 1 shall now 

 proceed to shew how an excellent trick may be performed with 

 the construction I have recommended, which cannot be played 

 with a Gregorian, because its concave metal is not parabolic. 

 Take out the erecting eyepiece and close up the hole with a 

 plug, remove the round plane, and substitute a diagonal one 

 with the necessary adjustment to the side of the tube, and 

 there is a little Newtonian sweeper, which will beat any Gre- 

 gorian of the same aperture on celestial objects. I conceive 

 that both of these constructions will be equally good with any 

 power they will bear. The Newtonian, perhaps, will carry 

 120 ; of course, the erecting eyepiece may be applied to it also, 

 if convenient. I have subjected this instrument to a species of 

 trial, by making an exact drawing of it of the full size, from 

 which the dimensions I have given have been taken.* On pa- 

 per, it seems feasible enough to me, yet I am well aware, that 

 some unforeseen circumstance may arise in the execution which 

 may utterly ruin its performance. 



I must protest against Sir D. Brewster's treatise on the 

 microscope, in the Encyclopaedia Britannica, as containing any 

 accurate account of my writings and doings where it professes 

 to do so. I select the following instance as an example. 



In the very first page of the treatise, it is represented in a 

 manner which no one, I think, can mistake, that I want to 

 quash the word " Microscope" altogether, and substitute for it 

 the term " Engiscope." Nothing can be more incorrect, as any 



* I do not think it worth while to give this drawing on a small scale, be- 

 cause it is so easy to imagine it. We have only to make the convex metal 

 of a Cassegrain plane, and to thrust a small erecting eyepiece through the 

 hole in the concave metal far enough in to meet the image, and there it i$. 



