58 Dr Goring on Monochromatic Light, Sfd 



otherwise than by drawing a line of demarcation between them, 

 by reference to the length of the anterior and posterior conju- 

 gate focus of the object-glass. Thus we may say that a telescope 

 has its anterior conjugate focus, or the distance of the object 

 from the object-glass longer than the posterior, or that next 

 the eye-piece. In an engyscope the case is reversed.* The line 

 of separation is therefore where the two foci are equal ; such 

 being the case, we may make an object-glass of short focus, 

 say four inches, gradually pass through all the gradations from 

 a telescope to a compound microscope, without making the 

 least change in the principle on which the image is formed. 

 Now, no optician has ever yet dreamed of making an achro- 

 matic telescope by arranging a system of glasses at particular 

 intervals, though, if a compound achromatic microscope or 

 engyscope can be so made, we ought surely to be able to con- 

 struct a telescope of some sort or other on the same principle, 

 the two instruments, as before stated, being so similar in 

 their action of forming an image. It is a consideration also 

 worthy of being attended to, that the first practical opticians 

 in Europe have now all adopted the method of making achro- 

 matic engyscopes precisely on the principle of telescopes, — 

 that is, by correcting the objective part by means of concaves 

 of flint-glass. Such men as Fraunhofer, Utzneider, Amici, 

 Selligue, Chevalier, Dollond, Tulley, would scarcely have all 

 adopted so difficult a task as that of making these minute ach- 

 romatic glasses, if it had been possible to obtain a perfect cor- 

 rection of chromatic and spherical aberration without them. 

 They are all in the habit of making so called achromatic, 

 erecting eye-pieces for spy-glasses, (which are in fact nothing 

 but compound microscopes ;) and it is hardly credible, that 

 they should not have perceived the application of them as such, 

 if they were, bonajide, achromatic. But the fact is, these ar- 

 tists all know the fallacy of these things, and how, and under 



* Perhaps a definition of an engyscope, as distinct from a telescope, may 

 be given thus : — It is a telescope made on a very small scale, having its ob- 

 ject-glass and the conjugate foci thereof reversed. It may not be generally 

 known that an engyscope will act with a concave eye-glass, just as a Gali- 

 lean telescope does, and that its focus can be adjusted by moving the eye- 

 glass alone, instead of the whole body, as is usually practised. 



1 



