NATURAL PHILOSOPHY. 225 



and the great depth of their curves ; and I have no doubt that, however im- 

 perfect, it is one which is anxiously and carefully applied. You arc, no 

 doubt, acquainted with Dr. Wollaston's interesting paper " On the Concen- 

 tric Adjustment of a Triple Object-Glass," (Phil. Trans., 1822, p. 32) 45 

 inches in focal length, executed by the celebrated John Dollond, and regard- 

 ed as one of his best works. By a process which he has described, Dr. 

 "Wollastou found that it was very imperfectly centered ; and, contrary to 

 the advice of his friends, he separated the lenses, and by applying two pairs 

 of adjusting screws to the edges of each lens, he placed their axes in the 

 same line, and, to use his own words, " he restored his object-glass to such 

 correct performance" that it was "capable or either separating very small 

 and nearly equal stars, as those of forty-four Bootis and <r Coronce, or of ex- 

 hibiting the minute secondaries of /3 Orionis and twenty-four Aquihc, with as 

 much distinctness as the state of the air would admit." Dr. TWilaston 

 adds, " that the actual limit to its powers cannot be fully ascertained, ex- 

 cepting under such favorable conditions of the atmosphere as do but rarely 

 occur." If such a distinguished artist as Dollond failed in centering a 

 group of three lenses, about four inches in diameter, and with comparatively 

 flat curves, how much more difficult must it be to center the six minute 

 lenses of an achromatic object glass one-eighth or one-twelfth of an inch 

 in focal length : and if such results were obtained by the correction of his 

 error, how superior must the microscope be in which the concentric adjust- 

 ment of its lenses is effected ? AYhilc opticians, indeed, confine themselves 

 to the use of only two kinds of glass, of different refractive and dispersive 

 powers, we can hardly expect much improvement in the microscope, unless 

 by the substitution of achromatic lenses in the eye-piece, and by an infalli- 

 ble method of centering each lens, and each group of lenses in the instrument. 

 The successful application of two pairs of adjusting screws to each of six 

 lenses, and also to those of the eye-piece, may be a difficult task, but it is 

 not beyond the powers of mechanism. It is very obvious that Dr. "Wallas- 

 ton's method of examining the centering of a triple object-glass is wholly in- 

 applicable to the object-glass of a microscope. In submitting to examina- 

 tion an object-glass made by a distinguished optician, it was necessary to 

 use a microscopic picture of the sun, and to examine the position of its 

 images as reflected from the various surfaces of the lenses by means of a 

 microscope, the object-glass of which was brought in contact with the outer 

 lens of the object-glass to be examined. By separating the two object- 

 glasses, I observed in succession a series of twenty-four images appearing 

 and disappearing in succession. These images occupied different parts of 

 the field, and I could not succeed by the most careful adjustment of the ap- 

 paratus employed in placing them in the same axis. These images had 

 various sizes, and were in various states of color, some highly colored, and 

 pome purely white. They had also various sizes, many with fine planetary 

 discs, of different magnitudes, some like the smallest fixed stars which it 

 was difficult to descry, and almost all of them exhibiting the most beautiful 

 concentric diffracted rings when put out of focus. Two or three images 

 often appeared in the same part of the field, in immediate succession, while 



