A. A. C. ELIOT MERLIN ON THE MINIMUM VISIBLE. 387 



minute spherule should be capable of exhibiting the same 

 recherchcs phenomena as a delicate glass lens l/30th focus solely 

 from its refractions and chromatic aberrations, at first seems 

 quite incredible." In another place * Dr. Poyston-Pigott con- 

 tinues : " The existence of dark eidolic dots of interference is an 

 important fact which now requires further elucidation. Darkness 

 has resulted from excessive light. Wave neutralising wave, certain 

 undulations killed each other. This is seen on a grand scale in 

 the solar spectra formed by a small lens in the foci of a very fine 

 microscojDe. Forty-eight dark rings have been counted developed 

 by an extremely small solar beam. The feeble refractions occur- 

 ring in a diatomic convexity cannot develop a very numerous 

 retinue of rings ; but sufficient diatomic lenses have been accumu- 

 lated for the purpose indicated. To exhibit successfully a series 

 of eidolic dots of interference demands very careful illumination 

 and a very fine objective. Their size varies with the nature and 

 diameter of the refracting spherule. The 1/Sth water lens of 

 Powell and Lealand seems to excel all my others in detecting 

 them in different focal planes. Six have been in order thus seen, 

 but in small spherules such as those of P. angtdatum many dots 

 are too faint for recognition. My experience of scale molecules 

 has convinced me they also are wonderfully transparent, display 

 black marginal test rings, and often one eidolic dot." . . . " These 

 dots are well developed by large beading of diatoms from 1/9, 000th 

 to 1/1 4,000th of an inch in diameter. Extremely large spherical 

 beads are seen in Cresswellia superba and in Cestodiscus superbus 

 (beads 1/1 2,500th) ; E. costatus and C oscinodisens radiatus are 

 also fine examples. To exhibit successfully all the eidolic dots of 

 interference in successive focal planes demands very excellent 

 glasses, careful precautions, and, above all, well-separated diatomic 

 beads. They may be caught above very small diatomic and scale 

 beading. Remarkably good eyesight has distinguished them 

 above the bosses of P. angulation and occasionally I have 

 detected two sets of dots when one stratum of beading lies just 

 below another. In general, except in strongly pronounced 

 diatomic bosses, the observer may rest satisfied with finding the 

 primary eidolic dot, No. 1, fig. 1 in the diagram. f A better glass 



* English Mechanic, vol. xlix. p. 315. 



t A diagram showing a series of eight gradually diminishing dots is 

 annexed to the original paper. 



