72 Dr Goring on Monochromatic Light, Sj-c. 



I shall now give the construction of an erecting eye-piece 

 or engyscope, which is both achromatic and aplanatic to an 

 aperture of about 27°, and having, moreover, a reasonable 

 good correction of the oblique pencil. It is represented by Fig. 

 1. Plate I. a and b are two achromatic lenses ; the focus of 

 b is to that of a as two to one, and the interval between them 

 is also equal to one ; c is a stop placed immediately behind b, 

 to regulate the aperture ; a may be over-corrected for spheri- 

 city and dispersion ; b under-corrected * for both, in such a 

 manner that their aberrations shall neutralize each other, or 

 each lens may be perfect in itself; d e is an Huygenian eye- 

 piece. The interval b d must be regulated according to cir- 

 cumstances ; but may be roughly stated as equal to that be- 

 tween the second and third glasses of a common erecting eye- 

 piece of the same power, -f- 



I have already stated that I consider the ordinary erecting 

 eye-piece sufficiently perfect for practical purposes; but I think 

 it would be a great improvement on spy-glasses if they could 

 be shortened nearly one-half, so that they should be easily 

 held and directed by the hand, when charged with a power of 

 thirty or forty. I think an aperture of two inches and a half 

 or three inches does as much upon terrestrial objects, at least 

 in the day-time, as any other, the focal lengths of which are 

 generally not less than thirty inches, which render the glass 

 too long to be used without a stand. If the focal length was 

 reduced to fifteen inches, the angular aperture would thereby 

 be doubled, and with it the defining power also, (provided it 

 could be perfectly executed.) In this case the erecting eye-piece 

 I have given would be indispensable, for the colour of an or- 

 dinary one would become very sensible. Moreover, when we 

 apply an erecting eye-piece to the object-glass of an engyscope, 

 so as to obtain a low power, and for that end thrust it far in- 



* Mr Lister has ascertained that an achromatic glass, having its inner 

 surfaces in contact will vary in its spherical aberration according to cir- 

 cumstances. It matters not how or in wh;it manner, or upon what princi- 

 ple, the aberrations of the two lenses are balanced. 



| When this construction is used as an engyscope, there will be an 

 abundant space left in front of the glass a for the illumination of opaque 

 objects, provided its focus is not shorter than T %ths of an inch. 



