']rn'il.Ap,'i?T.T^''] Hujh-power Definition. 193 



besides tlie focal images of tlie Formosa and Angulata beading, 

 there emanates a rich bundle of iridescent rays from all these won- 

 derful little sihceous gems; small though they be, they are huge 

 compared with the wavelets that play around and within them. 

 As the falling drop refracts the sunbeam, reflects it twice internally 

 before returning it to the outer air and changes its colours simply 

 by new deflections, so the diatom beading plays with the ethereal 

 particles, dispersing its spreading rays in full prismatic glory. 

 Yet these colours, so evident even to the unassisted sight en masse, 

 vanish in achromatic vision! Should then these charming cha- 

 racters be destroyed? Dechromation of Nature's own colours 

 resplendent in beauty is a chromatic crime. 



The colouring of many objects is more exquisite and fascinating 

 as the perfection of high-power definition advances. We have a 

 wonderful example of this in the recently-discovered colouring of 

 Jupiter's disk, which can never be as well seen by achromatic — 

 artificially achromatic glasses — as by the non-chromatic mirrors. I 

 have lately seen the Formosum beading coloured with red, orange, 

 yellow, and blue. The beading has appeared wreathed with a 

 golden bronzing. Individual beads separated from their fellows 

 have appeared remarkably distinct. Destruction of colour reduces 

 the field to a spiritless picture. Objects which ought to be and, in 

 some cases, can be, lit up with a startling brilliance of hue, regain 

 with altered corrections the tame colouring of a dull prevaihng 

 yellow and black. There is something recondite here beyond our 

 ken, to be patiently searched and imder stood. 



The focal image of a brilliant flame may be distinguished just 

 above a spherical bead, in general only as a round disk. All change 

 in the shape of the flame is useless. The brilliant focal disk may 

 be found just above the surface of the bead. But I have observed 

 this singularity about it, that it expands the bead into an abnormal 

 size. Thus in observing the Podura headings which are excessively 

 transparent and refractive, wherever a bead (considered as a refracting 

 lens) is so situated among the crossing rouleaus that its full aperture 

 allows a brilliant focal point to be formed (according to the direction 

 of the illuminating ray), there, it acquires nearly double its real size 

 and an exaggerated importance, and of course tells a delusive tale. 



It is probable that the Diatom beading I now beg to urge our 

 Fellows to examine, will finally give up the focal images, of a 

 black cross (say a Maltese one), to an aperture varying from sixty 

 to ninety degrees according to the refractive index of the siliceous 

 spherules themselves. It can be readily demonstrated that aperture 

 has a peculiar efiect upon the action and reactions (if I may so 

 speak) of refracting particles upon the hght and pencils introduced 

 to the eye of the observer by the observing objective. I would just 

 remark that the focal images of convex lenses being inverted twice, 



VOL. III. o 



