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ApocJiroinatic Object- Glasses. [Sess. 



has its own focal point, depending upon its refractive power, so 

 each one must of necessity have a separate magnifying power 

 with the same lens. This, it will be seen, introduces a very 

 strong element of confusion in the image rendered by an 

 achromatic lens of the very best construction. lu the con- 

 struction of an achromatic lens any two of the rays might be 

 taken for combination. The lens might be, as many are, 

 corrected for the red end, or give a very red image ; or they 

 might be corrected for the violet end, as is done in photo- 

 graphic lenses. But the point to be kept prominently in view 

 is, that whatever two were taken, there could only two be 

 taken, and the others left. 



Now, taking that as the principle of construction of the 

 ordinary achromatic lens, we shall see why so much has been 

 done to get rid of the " secondary spectrum," and the effect it 

 has always had in the deterioration of the image in the best 

 constructed lenses. We have heard of it specially in the 

 construction of object-glasses for telescopes, in the best of 

 which it has usually made itself apparent, declining to be left 

 out of view. The object in view in the construction of the 

 new apochromatic lenses has been, to get rid of this secondary 

 spectrum, and to combine at least four of the refracted rays, 

 so as to produce a still further perfected image, and render 

 the other outstanding and uncombined rays, termed the 

 " tertiary spectrum," of little, or at least of much less, account 

 in the formation of the image. This object has been sought 

 to be attained by two means — first, by the formation of 

 certain kinds of very dense glass containing other substances 

 than lead, which has been used for density, in the con- 

 struction of the object-glasses ; and second, by the use of 

 specially constructed eye-pieces to correct still further the 

 effects inherent in the flint-glass. It is unnecessary here to 

 detail the nature and kinds of glass used. These may be 

 found in detail in the printed pamphlet issued by the makers 

 of the glass. They have been able to construct glass having 

 a density very much greater than that of the diamond. The 

 makers of the new apochromatics have of course kept their 

 methods of construction a secret, and we know nothing as to 

 the formulae upon which they are constructed, and can judge 

 them only by the results they give us with known tests. 



