On the Achromatic Eye-pieces of Spy glasses, 269 



walking, which often, together with the lengthening of the 

 bone, is almost the only sign by which the commencing com- 

 plaint is made known externally. 



On an Arrangement in the Constrtiction of the erecting of 



Achromatic Eye-pieces of Spy-glasses, whereby their magni- 



,fy'ing 'power may always he adapted to the state of the atmo- 



sphere. By Charles Goring, M.D. Communicated by the 



Author. 



The artists of Vienna and Paris having adopted the princi- 

 ples laid down by me for making the eye-pieces of spy-glasses, 

 hona fide achromatic, as originally given many years ago in 

 a paper in the Edinburgh Journal of Science, and afterwards 

 more fully developed in a tract in the " Micrographia,""* I 

 avail myself of the circulation this journal has on the Conti- 

 nent to suggest an improvement upon this description of eye^ 

 pieces, which will, I think, be found highly convenient and 

 useful ; — {to those of the common construction it cannot he ap" 

 plied with effect and advantage). 



They who are acquainted with my writings will not require 

 to be informed that the doctrines I have promulgated on the 

 construction of achromatic erecting eye-pieces arc, that they 

 are neither more nor less than compound microscopes, applied 

 to the purpose of magnifying the image of the primary object 

 glass, and that if they are to be really free from chromatic and 

 spherical aberration they can only be made so by the operation 

 of concaves of flint glass applied to the secondary object-glass, 

 or that of the compound microscope. It is well known that spy- 

 glasses are made of much greater length than can be conve- 

 niently held and directed by the hands, and that, in conse- 

 quence, if their power is considerable, the object dances on the 

 retina in such a manner that it is impossible to examine it, and 

 the goodness of the glasses is thus nullified in practice. My 

 object has been to shorten them by combining two primary ob- 

 ject-glasses together, to double their angular aperture while 

 their focal length is reduced one-half, thus rendering them 

 dumpy, short, and thick. If this is done, the necessity of 



* On the chromatic and spherical aberration of eye-pieces. 



