4-10 Dr. Royston-Pigott on [June 19, 



particles, monochromatic light, and the view of a higher focal plane than 

 that which showed the markings . 



B. — Eyepieces t 



The spherical aberration of positive eyepieces may be examined as 

 follows :— 



The positive in an inverted position is placed under the stage, and 

 forms an image of the solar disk for examination. 



DESCRIPTION OF THE PLATES. 

 Circular Solar Spectrum. 

 Plate II. 



Fig. 1. Primary rings finely defined at the first risible focal plane. 

 Tig. 2. Secondary rings, at a deeper focus. The central disk should have been squared 

 off. 



Pig. 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 suc- 

 ceeding 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 irregular disks 

 showing the existence of displaced centres and irregular diffractions. 



Plate III. 



Figs. 8, 9. The miniature 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 



Beek, who is spending some months in the United States. He exhibited to me (with his 

 fine high-power objectives) several slides of the scale of the Lepidocyrtus curvicollis 

 [test-scale Podura] collected by himself from carefully identified insects. He aimed 

 at the exhibition of the exclamation-marks, as such, using a right-angled prism, achro- 

 matic condenser, and small coal-oil lamp. Before leaving Washington he was so good 

 as to give me one of these slides, indicating on it a scale which appeared particularly 

 adapted to the display of the exclamation-marks. Certainly this scale, which was 

 ■jjfjs of an inch in length and T ^ broad, surpassed in the distinctness of its mark- 

 ings any of the scales of this species which have hitherto come into my possession. 

 With central monochromatic light, the immersion and amplifier, I obtained a nega- 

 tive showing the exclamation, marks better than any representation of the kind I have 

 yet been able to obtain. I send herewith a paper print. The magnifying-power is 

 3200 diameters." (Dr. Woodward also favoured the writer with copies.) 



" But immediately afterwards, with the same optical combination and magnifying- 

 power, without any change in the cover correction, by simply rendering the illumi- 

 nating pencil oblique, and slightly withdrawing the objective from its first focal position , 

 I obtained a negative which displays the " bead-like " or varicose appearance of the 

 ribbing more satisfactorily than I had previously been able to do." (Month. Micr. 

 Journ., June 1871, p. 245.) [The writer has italicized the quotation.] 



