412 Dr. Goring on Achromatic Microscopes, 



what concerning the proper length of the body of an 

 achromatic microscope ; but I am afraid it will be impossible 

 to determine the problem with any positive accuracy. From 

 experiment it would appear, then, that the length of the an- 

 terior focus of the object-glass should bear some certain ratio 

 to the other conjugate focus, in order that the instrument may 

 do its utmost. Thus, an object-glass of long focus requires a 

 longer body than one of shorter focus ; but it is almost im- 

 possible to lay down the precise dimensions which answer or 

 ought to answer best : we may, perhaps, reasonably assume 

 the maxim, that the object-glass will work best along with 

 that body which was annexed to it at its formation, and 

 along with which it was originally made to correct. It 

 may be asserted, I think, that an aplanatic of 0*9 focus 

 will not act well with a body of less length than 9 or 10 

 inches ; while one of 0*2 focus seems to do its best, or at least 

 do perfectly well, with one of about 6 inches or even less. 

 The tubes may be increased in length beyond this mark, 

 however, without making any very sensible or decided alter- 

 ation in performance. We might expect, that as the radiant- 

 point always approaches nearer to the object-glass in propor- 

 tion to the elongation of its anterior focus, we should gain 

 somewhat by the slight increase in the angle of aperture 

 which is thus occasioned, while opaque objects would be seen 

 better, because their light being radiant and not of the inter- 

 cepted kind, is diminished or increased in proportion to the 

 square of their distance from the object-glass. But it does 

 not seem to me that the human eye is at all able to appre- 

 ciate the difference in the performance, if any. On the other 

 hand, long tubes are very inconvenient ; they place the eye 

 of the observer at a very disagreeable distance from the 

 stage of the instrument, which can thus hardly be used with 

 comfort for dissection in a vertical position : they moreover 

 require steadying-rods and other contrivances to give them 

 the requisite stability. It may be suspected^ moreover, that 

 their rays in travelling over a large space (if they have any 

 error in them) will become still more erroneous^ and form a 

 worse image than they would if cut short at a less length. 

 It is well known, that Sir I. Newton invented his telescope. 



