Dr Goring on Monochromatic Light, fyc. 65 



piece such achromatism is absolute and perfect, and not like that 

 effected by the combination of a concave lens of flint-glass with 

 a convex of plate or crown-glass, which never effects a com- 

 plete neutralization of the chromatic aberration, as is well 

 known. Therefore, if engyscopes and telescopes could be made 

 by the adjustment of a system of lenses to particular intervals, 

 their achromatism ought to be decidedly superior to that ob- 

 tained by the action of concaves of flint-glass, and equal to 

 that of a reflecting instrument. 



2. The only kind of achromatisation produced by convex 

 lenses which is known in practice, is when two are adjusted to 

 an interval equal to one-half the sum of their focal distances, 

 or thereabouts. (These conditions are not rigorously neces- 

 sary, as the lenses may be placed somewhat nearer than that.) 

 Moreover, the intervals seem to differ a little according as the 

 eye-piece is adapted to an engyscope or to a telescope, — at least 

 if the power of the former is low and the tube short. 



3. Many modifications of this combination may be made, as 

 by doubling or tripling what are usually termed the eye and 

 field-glasses, so that the compensation for a double or triple 

 eye-glass may be thrown upon a double or triple field-glass, 

 and that there may or may not be an interval between the 

 lenses composing the eye and field-glasses ; still, however, the 

 mode of operation is the same. 



4. In order to form a truly achromatic erecting eye-piece 

 or engyscope, there must be a compensation both in that part 

 which erects and forms the image, and in that which views it ; 

 therefore no achromatic erecting eye-piece or engyscope can 

 be made with so few as three lenses, because either the objec- 

 tive or ocular part must inevitably be without compensation. 



5. An erecting eye-piece can only be made really achroma- 

 tic, (if we do not employ concaves of flint-glass,) by combin- 

 ing two Huygenian eye-pieces, or some modification thereof, 

 together, in such a way that one shall erect and form the image, 

 while the other is made to view it. As to the interval between 

 them, or what may be termed the second and third glasses, if 

 the eye-piece consists of four, it may be greatly varied without 

 sensibly affecting the achromatism. 



6. Such an eye-piece could only be used for viewing an 

 image, and could never be employed as an engyscope, because 



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