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II. — Researches in Circular Solar Spectra, ai:)])Ued to test 

 Residuary Aberration in Microscopes and Telescojpes ; * 

 and the Construction of a Comi)ensating Eye-inece, being a 

 Sequel to the Paper on a Searcher for Ajplanatic Images. 

 By Gr. West Eoyston-Pigott, M.A., M.D. Cantab., Memb. 

 Koy. Col. Phys., Fellow of the Camb. Phil. Soc, the Eoyal Ast. 

 Society, &c., and late Fellow of St. Peter's Coll., Cambridge. 



(^Communicated hy Prof. Stokes, Sec. E.S., to the Eoyal Society. Eeceived 



April 24, 1873.) 



Plates XXXIII., XXXIV., XXXV., and XXXVI. 



The researches detailed in the present paper were commenced in 

 May, 1871. The results arrived at were largely obtained from 

 using the microscope. Similar but less brilliant and more scanty 

 appearances can be obtained with the telescope ; but the very high 



EXPLANATION OF PLATES XXXIII., XXXIV., XXXV., AND XXXVI. 



Circular Solar Spectrum. 

 Plate XXXIII. 

 Fig. 1. — Primary rin2,s finely defined at the first visible focal plane. 

 „ 2. — Secondary rings, at a deeper focus. The central disk should have been 



squared off. 

 „ 3. — Exhibition of the greatest display of rings, each annulus having its own 

 breadth equal to that of the central disk, which is a brilliant white, 

 the succeeding lavender-rose colour and red rings separated by dark 

 rings, the first few of which are jet-black. 

 Figs. 4, 5, 6, 7. — Development of two disks instead of one ; also of four and irre- 

 gular disks showing the existence of displaced centres and irregular 

 diffractions. 



Plate XXXIV. 

 „ 8, 9. — The miniatm-e prospect of the distant window is displayed sharply in 

 Fig. 8. So soon as the sun began to shine, the blazing prism being 

 quenched by turning it aside, every brilliant point became irradiated 

 with an orange-red halo (Fig. 9). If the coloiu- were corrected by 

 change of the general adjustments so as to destroy halo, then the 

 prospect in Fig. 8 became enveloped in a strong wliite mist of uncor- 

 rected residuary aberration. 

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

 glasses, are here well rejDi'esented. 

 Fig. 12. — Displays the very delicate engine-turned pattern and obscuration of the 

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

 „ 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 delineated. 



Fig. 14. — Shows the blurred appearance of the rings when the spherical aberration 

 is excessive ; similar also to the evanisliing spectrum occurring before 

 the fog appears. 



. * That part of the paper referring to the new correction for telescopes is 

 omitted. 



VOL. X. P 



