Dr Goring mi Monochromatic Light, $c. 71 



corrected by giving the small one an hyperbolical figure. More- 

 over, in the Cassagrainian telescope, it is well known that the 

 aberration of the two metals (supposing them to be both spherical) 

 is equal to the difference between the aberration of the convex 

 and the concave, which is a clear proof that an eye-piece may 

 correct to a certain extent ; (for I consider that the small metals 

 and eye-glasses of the Gregorian and Cassagranian telescopes, 

 in their combined action, are neither more nor less than eye- 

 pieces, which operate by forming a secondary image like the 

 erecting ones of common spy-glasses.) I should, moreover, 

 conceive that the same conditions would occur if achromatic 

 glasses of similar force, and in equivalent states of correction, 

 were substituted for the metals of the Gregorian telescope, 

 and made to operate against each other on the same principle ,• 

 but this experiment I have never tried, either in telescopes or 

 engyscopes. 



I have in vain endeavoured to find an inverting eye-piece 

 which would exert an influence over the state of spherical 

 aberration of the object-glasses and metals of engyscopes. My 

 way of judging was to form an image of an artificial star by 

 their unassisted operation, as magnifiers in a solar opaque mi- 

 croscope, making the posterior conjugate focus to be of the 

 same length as in an engyscope, that is, eight or nine inches, 

 and having carefully examined the state of aberration in the 

 picture, I then have attempted to modify it (when formed at 

 the field bar of an engyscope) by various eye-glasses ; but so 

 far as my eyes are capable of affording a criterion, I never 

 succeeded in any degree. The image appeared as intractable as 

 a picture formed by human hands, and remained inflexible 

 both in its defects and excellencies, whatever they might be. 

 The various eye-pieces I employed gave a better or worse field 

 of view, were or were not achromatic, &c. as regarded their 

 own intrinsic operation ; for example, they might give the ob- 

 lique pencil in a better or worse state ; but if the said oblique 

 pencil came in a confused or distorted state from the object- 

 glass, I never could make them improve it. At the same time, 

 I think it proper to observe, that 1 never tried thoroughly the 

 effect of positive achromatic eije-picc.es in which there wtU a 

 correcting concave lens qf'Jlint-glass. 



