THE 



MONTHLY MICROSCOPICAL JOTRNAL. 



JULY 1, 1873. 



I. — Observations on the Optical Ap'pearances presented hy the 

 Inner and Outer Layers of Coscinodiscus ivhen examined in 

 Bisidyhide of Carhon and in Air. By J. W. Stephenson, 

 F.E.A.S., Treasurer E.M.S., and Actuary to the Equitable 

 Assurance Society. 



(Read before the Eoyal Microscopical Society, June 4, 1873.) 

 Plate XXI. (Upper portion). 



The possibility of determining tbe structure of minute organisms by 

 means of the refractive indices of the media in which they are ex- 

 amined has been alluded to on more than one occasion, and notably 

 in the discussion which followed Mr. Slack's paper on Eupodiscus 

 Argus, when Mr. Charles Stewart stated that the silicious deposits 

 in both plants and animals are of less refractive index than Canada 

 balsam, and that consequently, when mounted in that medium, they 

 appear, if convex, to act as concave lenses do in air, and vice versa. 



In fact, that gentleman has been in the habit, in his investiga- 

 tions of the Echinoderms, of determining the nature of the spicules 

 entangled in the tissues of those animals by the apjiearances pre- 

 sented when mounted in Canada balsam. The refractive index of 

 Canada balsam is higher than that of silicious, and lower than that 

 of calcareous, spicules ; consequently, there is no possibility of con- 

 fusing the calcareous spicules of the Echinoderm with those of the 

 silicious sponges on which the creature may have been feeding. 



It was long since pointed out by Welcker as a means of 



EXPLANATION OF PLATE XXI. (Upper portion). 

 Coscinodiscus occulus iridis, in bisulphide of carbon. 



Fig. 1. — Hexagonal areola of inner or " eye-spot " layer, as seen when beyond the 



fucus. 

 „ 2. — Ditto beyond, but nearer focus than Fig. 1. 

 ^-==' „ 3. — Ditto, supposed true focus, showing fracture through " ej^e-spot." 

 ^P „ 4. — Ditto, within the focus. 



5. — Areola of cellular or outer layer, slightly beyond the focus. 

 6. — Ditto, ditto, sliglitly within focus. 

 7. — Both layers, beyond focus. 

 r— { „ 8. — Ditto, beyond, but nearer focus than Fig. 7. 



„ 9. — Ditto, supposed true focus. 

 Ci5 „ 10. — Ditto, within focus. 



'-'-' „ II. — Ditto within, nearer true focus than Fig. 10. 

 ^^ „ 12. — Ditto within, nearer true focus than Fig. 11. 



VOL, X. B 





