THE MIOBOSOOPB. %\) 



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 of the microscope is then perfected, if 

 necessary, 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 disturbing 

 the correction, opposite errors which balance each other being 

 produced by the change. 



"If the two glasses which in the diagram are drawn at some 

 distance apart are brought nearer together (if the place of A, 

 for instance, is carried to the dotted figure), the rays trans- 

 mitted by B in the direction of the longer aplanatic pencil of A 

 will plainly be derived from some point Z more distant than 

 F", and lying between the aplanatic foci of B; therefore (ac- 

 cording to what has been stated) this glass, and consequently 

 the combination, will then be spherically over-corrected. 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 color may in the same manner be destroyed 

 by opposite ones; and on the principles described we not only 

 acquire fine correction for the central ray, but by the opposite 

 effects at the two foci on the transverse pencil, all coma can be 

 destroyed, and the whole field rendered beautifully flat and 

 distinct. " 



Mr. Lister's paper enters into further particulars, which are 

 not essential to the comprehension of the subject. It is suf- 

 ficient to say that his investigations and results proved to be 

 of the highest value to the practical optician, and the progress 

 of improvement was in consequence extremely rapid. The new 

 principles were applied and exhibited by Mr. Hugh Powell and 

 Mr. Andrew Boss with a degree of success which had never 

 been anticipated; so perfect indeed were the corrections given 

 to the achromatic object-glass so completely were the errors 

 of sphericity and dispersion balanced or destroyed that the 



