1873.] Circular Solar Spectra 4U 



(fig. 9). If the colour were corrected by change of the general adjustments 

 so as to destroy halo, then the prospect in fig. 8 became enveloped in a strong 

 white mist of uncorrected residuary aberration. 



Pigs. 10, 11. The slight deviations from the true circular form, owing to imperfect 

 glasses, are here well represented. 



Fig. 12 displays the very delicate engine-turned pattern and obscuration of the diffrac- 

 tion-rings by a badly constructed glass (cheap German). 



Fig. 13 shows a delicate set of rings between the coarser, expanding by a change of 

 focus in a different manner. The approaching halo and fog are well deli- 

 neated. 



Fig. 14 shows the blurred appearance of the rings when the spherical aberration is 

 excessive ; similar also to the evanishing spectrum occurring before the fog 

 appears. 



Plate IV. 



Fig. 15. In this case the heliostat was placed nearly forty feet distant. The internal 

 lenses of a fine £ objective being all removed, the thick front only was employed 

 to form the miniature on the stage. A peculiar irregularity in the central jet- 

 black rings is supplemented by extraordinary excentric lines bordered by a 

 new order of peripheral rings, obeying a different law of expansion. Viewed 

 under Powell's best dry -i, a half-inch single plano-convex lens being used as 

 eyepiece. 



The next figures illustrate the effects of obliquity, the previous drawings exhibiting 

 various effects during coincidence of the axis of the microscope with that of the miniature- 

 forming lens. 



Figs. 16 & 17 represent a very beautiful variety of hyperbolic diffraction-lines seen 

 when the axis of the solar ray is inclined about 6 degrees to that of the 

 microscope. Distance of heliostat 20 feet, magnifving-power 1000. Powell's 

 best £ forms the miniature observed with best dry eighth. 



Fig. 17. Miniature objective, a ~g- Gundlach immersion used dry. Microscope ob- 

 jective best ^ dry eye-lens, piano half-inch, parabolic curves and fine dif- 

 fractions, obliquity 5 degrees. 



Fig. 18. The appearance within the focus of the best and first resolution of the fog of 

 undercorrection. 



Fig. 19. Slight obliquity and undercorrection. 



Plate V. 



Figs. 20, 21, 22. Best £ and J plano-concave stage-lens. 



The circular spectra here delineated are produced by slight obliquity, and represent 

 the appearances at different focal planes. The colours are extremely brilliant, and the 

 lines perfectly sharp in their tracery. 



Fig. 23 shows the lines formed by the circular solar spectrum viewed with the greatest 

 obliquity attainable, the elliptic lines representing a plane cutting both side 

 of the cone of converging rays. 



Figs. 24 & 25 introduce a new order of figures formed by placing a mercurial globule 

 10 inches from the stage, and placing the 1^-inch objective condenser with its 

 axis considerably inclined (an angle of 15°) to that of the microscope; the 

 images of the solar disk then took these and the forms in figs. 26-34, as seen with 

 a microscope armed with a good eyepiece (Kellner 1-inch) and a fine i objec- 

 tive. 



