172 Besearclies in Circular Solar Spectra. 



power and ready adaptability of the former confers some advan- 

 tages not offered by the latter. 



In both, however, the same principles are illustrated. 



A cone of rays of small angular aperture having the object- 

 glass for its base, in each case engages the eye-piece and emerges 

 parallel, and the eye-pieces are similar in each. 



Peculiar facilities for studying solar spectra and their indi- 

 cations of aberrations and mechanical errors also are afforded by the 

 former. The focal plane of vision may be employed to examine 



Explanation of Plates — continued, 



Plate XXXV. 

 Fig. 15. — In this case the heliostat was placed nearly 40 feet distant. The in- 

 ternal lenses of a fine J-objective being all removed, the thick front 

 only was ensployed to form the miniature on the stage. A peculiar 

 irregularity in the central jet-black rings is supplemented by extra- 

 ordinary eccentric lines bordered by a new order of peripheral rings, 

 obeying a different order of expansion. Viewed under Powell's best 

 dry ith ; a |-inch single plano-convex lens being used as eye-piece. 



The next figures illustrate the effect 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° to that of the 

 microscope. Dist. of heliostat, 20 feet ; magnifying power, 1000. 

 Powell's best \ forms the miniature observed with best dry ^th. 



Fig. 17. — Miniature objective ; a yLth Gundlach immersion used dry. Microscope 

 objective best ith used dry, eye-lens J-inch couvexo-plane ; parabolic 

 curves and fine diffractions ; obliquity 5"^. 

 „ 18. — The appearance witliin the focus of tlie best and first resolution of the 



fog of under-correction. 

 „ 19. — Slight obliquity and under-correction. 



Plate XXXVI. 

 Figs. 20, 21, 22. — Best ith and J plano-convex 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 witli the 

 greatest obliquity attainable ; the elliptic lines representing a jjlaue 

 cutting both sides 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 IJ-incli object con- 

 denser with its axis considerably inclined (an angle of 15") to that of 

 the microscoiie, as seen with a microscope armed with a good eye-piece 

 (Kellner 1-inch) and a fine J-objective. 



Very beautiful transformations of the circular solar spectra are seen by viewing 

 the solar disk of the mercurial globule portrayed obliquely on the stage, which 

 vary in their forms according as the glasses are under or over corrected, which it 

 is needless here to describe in detail. 



Tlie figures displayed by the magnified artificial star for oblique reflexion 

 render it probable that the obliquely illuminated mercury globule, viewed directly 

 in close proximity to the front glass of tlie microscope upon the fltage, is a very 

 imperfect test, and the methods here described are submitted as possessing very 

 superior delicacy and convenience. 



