Achromatic Eye-pieces (^Spy-glasses. 261 



termediate change of power, between 40 and 20 or 20 and 10 

 &c. may also be obtained at pleasure, to suit the state of the 

 weather. It will be necessary, however, to accomplish this pur- 

 pose tliat the two double achromatics should be perfect in them- 

 selves^ and the single moveable one also, otherwise the balance 

 of aberration will be liable to be disturbed by a change of posi- 

 tion. There is no stop in this construction to keep out false 



light, for it would be necessary to alter its aperture every time 

 the magnifying power of the glasses was changed, otherwise it 

 would be ineffective, or cut oft* something from the aperture of 

 the primary object-glass ; therefore, it will be necessary to fur- 

 nish the main tube of the telescope with stops at every joint, 

 embracing the cone of rays from the first object-glass, pretty 

 closely to within a few inches of the eye-piece, which will pre- 

 clude the necessity of a stop between the glasses, and be of the 

 greatest use when w^e view objects while the sun is low down in 

 the horizon, and nearly opposite to us. If stops must be had 

 between the said glasses, a wheel similar to that carrying the 

 concave eye-glasses of a perspective may be placed directly be- 

 fore the third glass, with holes in it corresponding to the size of 

 the image of the first object-glass as given by the different mag- 

 nifying powers, caused by pulling out or thrusting in the slid- 

 ing tube ; and when the observer has fixed upon his power, the 

 hole adapted to it must be rendered centrical by turning the 

 wheel round. In order to render the spy-glasses still more ca- 

 pable of being directed with firmness and precision towards an 

 object, and of preserving it steadily in the middle of the field 

 of view, I have recommended M. Chevalier to cause them to 

 be fastened at pleasure to a slight stock like that of a gun : 

 two straps with buckles will secure the glass as well as any- 

 thing ; and this would be no bad appendage to any spy-glass 

 not of an immoderate length. 



It must be obvious that when a system of glasses, such as I 

 have described, is made of long focus, it must form an excellent 

 Megalascopey or glass for viewing the larger kind of niicrosco- 



