1883.] 



MICROSCOPICAL JOURNAL. 



53 



matically undercorrected ; if the red 

 rays meet sooner than the violet, it is 

 said to be overcorrected. 



With the lower powers the artificial 

 star is about the best lest for de- 

 termining this condition. With the 

 glass now used by opticians for the 

 construction of objectives, perfect 

 achromatism has never been at- 

 tained ; there being at best a secon- 

 dary spectrum caused by the unequal 

 dispersion of the various parts of the 

 primary spectra of the Hint and crown 

 glass lenses. These colors are a pale 

 green and pale violet. If a well-cor- 

 rected objective be focussed upon the 

 mercury globule, and then brought 

 without and again within the focal 

 point, it will be seen that the circle 

 of light has a colored border, and 

 that when the globule is without the 

 focus this border is of a pale green 

 color, when within it a pale violet. 

 These colors indicate the best chro- 

 matic correction, and any variation 

 will show itself by more or less 

 change in the color. If undercor- 

 rection be present it is indicated by 

 the green taking on a bluish cast, or, 

 if very much undercorrected, a distinct 

 violet is to be seen. In the place 

 of the proper violet — that is when 

 within the focus — a reddis'h violet, 

 or even a brilliant red, is seen. Over- 

 correction is indicated when in the 

 place of the green, a yellowish-green, 

 brilliant yellow, or orange is seen ; 

 and in the place of the proper violet, 

 a blue border is seen. This applies, 

 not only to the mercury globule when 

 used as stated, but to almost any 

 opaque border of an object. With- 

 the higher powers, a very convenient 

 way is to use a well-marked diatom, 

 or, what in most cases is still better, 

 a strongly marked podura scale. In 

 any such objects as these, the dots 

 or spines appear violet and the inter- 

 spacing green when the chromatic 

 correction is right ; and in the case 

 of under or over correction these colors 

 give way to the colors already men- 

 tioned as occurring with the artificial 

 star. 



The correction of spherical aberra- 

 tion is more important than that of 

 color. By spherical aberration is 

 meant that the rays of light which 

 pass through different zones of the 

 objective do not unite in one point 

 in the eye-piece. If the peripheral 

 rays meet before the central ones, the 

 objective is said to be spherically un- 

 dercorrected, while if the reverse be 

 the case, overcorrection is present. 

 With the artificial star the presence 

 of spherical aberration is shown by 

 the coma, or circle of light, expanding 

 unequally when the globule is placed 

 within the focus, or the same dis- 

 tance beyond the focus. Should the 

 greater expansion of the coma take 

 place when the distance between the 

 objective and globule is increased, 

 the objective will be undercorrected, 

 but when the greater expansion is 

 within the focus, overcorrected. 



I have here this evening a one-inch 

 objective with which, by the aid of 

 the artificial star, I shall practically 

 illustrate the appearances seen with 

 a well-corrected glass. The work- 

 manship of this lens is superb, and, 

 although of moderate angular aper- 

 ture, it is much better in definition 

 and resolution than many lenses of 

 much wider angle. By the aid of the 

 back system of a cheap English one- 

 fourth-inch objective, I shall illustrate 

 the irregularity of outline of the coma, 

 which indicates a want of true form 

 or curves, of the lenses. 



Angle of aperture is of great im- 

 portance. By this is meant the an- 

 gular breadth of the cone of light 

 received from the object by the ob- 

 jective. This is to be determined by 

 direct measurement. As convenient 

 a method as an)', is by using a 

 stand which has a graduated rotating 

 base, or graduated swinging sub- 

 stage bar. The objective should 

 be put upon the stand and accurately 

 focussed, the stand being in a hori- 

 zontal ^ position. A lamp is then 

 placed several feet distant, and, by 

 turning the stand upon the base, 

 the aperture is ascertained, the light 



