£50 Mr Lister on the improvement of the 



combination shall be free from spherical error also in the cen- 

 tre of its field. For this the rays have only to be received by the 

 front glass B, Fig. 6, Plate I. from its shorter aplanatic focus/, 

 and transmitted in the direction of the longer correct pencil/ A 

 of the other glass A. It is desirable that the latter pencil should 

 neither converge to a very short focus, nor be more than very 

 slightly, if at all, divergent ; and a little attention at first to 

 the kind of glass used, will keep it within this range, the denser 

 flint being suited to the glasses of shorter focus and larger 

 angle of aperture. 



The adjustment for the microscope is then perfected, if ne- 

 cessary, by slightly varying the distance between the object- 

 glasses ; and after that is done, the length of the tube which 

 carries the eye-pieces may be altered greatly without disturb- 

 ing the correction ; opposite errors which balance each other 

 being produced by the change. 



If the two glasses, which in the diagram are drawn as at 

 some distance apart, are brought nearer together, (if the place 

 of A for instance, is carried to the dotted figure,) the rays 

 transmitted by B in the direction of the longer aplanatic pen- 

 cil of A, will plainly be derived from some point (2?) more dis- 

 tant than /", and lying between the aplanatic foci of B ; there- 

 fore (according to what has been stated) this glass, and conse- 

 quently the combination, will then be spherically over-cor- 

 rected. If on the other hand the distance between A and B 

 is increased, the opposite effects are of course produced. 



In combining several glasses together, it is often convenient 

 to transmit an under-corrected pencil from the front glass, 

 and to counteract its error by over-correction in the middle one. 



Slight errors in colour may in the same manner be destroy- 

 ed by opposite ones ; and on the principles described, we not 

 only acquire fine correction for the central ray, but by the op- 

 posite effects at the two foci on the transverse pencil, all coma 

 can be destroyed, and the whole field rendered beautifully flat 

 and distinct. 



The occurrence of two good combinations among the nume- 

 rous ones of Utzschneider's glasses will now appear only what 

 might be expected ; and more are to be obtained from them 

 by varying the distance of the glasses from each other. 



