1873.] 



Circular Solar Spectra. 



439 



Appendix. 



A. — Definition of min ute organic particles. 



A new fiftieth immersion lens, price thirty guineas, has been made for 

 me by Messrs. Powell and Lealand ; with this glass, without any obli- 

 quity, and using a tube four inches shorter than usual and a B eyepiece, 

 the upper continuous ribs of the Podura were resolved into strings of 

 blue sapphire-like beads appearing perfectly circular. The interspaces 

 between the markings at a lower focus showed lower strings of white 

 beads. 



Monads appeared blue, swimming about in the water used, and also 

 lying in well-defined masses ; some of them could be seen to rotate ; the 

 cilia were invisible, but the movements gave strong indications of them. 

 —April 21, 1873. 



A blue glass improved the definition. Coal-oil lamp and l|-mch open 

 condenser object-glass. 



• At present nothing is more difficult of definition in the microscope 

 than an assemblage of minute refracting organic particles. Virtually 

 forming disks of light, these display the diffraction-errors and phenomena 

 more or less vividly. No English microscopist has yet succeeded, so far 

 as is known to the writer, in displaying, in the apparently blank spaces j 

 the beaded structure seen between the celebrated exclamation-markings 

 of the Podura test-object. Here a closely packed mass of organic particles, 

 highly refractive and transparent, obscure each other ; brilliant points are 

 swelled out exceedingly, for a theoretical solar disk one millionth of an 

 inch * in diameter appeared as large as a disk the sixty thousandth. To 

 define accurately bright organic particles, such as those of the smallest 

 test-Podura, the molecules of cancer-cells and other diseased forms, and 

 monads (minute atoms of metal by reflected light might also be named), 

 is at present impossible ; when such delicate forms are in quest, all rays 

 of an aberrating character must necessarily be extinguished. 



Illustration. — Examining with an excellent new ^ immersion at 

 sunset, as already alluded to, it was only when a peculiar bluish-green 

 of the sky was thrown by an open excellent 1^-inch objective glass 

 condenser upon the stage that the internal organic particles of a deli- 

 cate Podura-sc&le became visible. Doubtless in this case the diffrac- 

 tion-spectra here had reached a minimumf. In the details given in the 

 accompanying note several points are particularly worthy of notice — the 

 use of an intermediate lens (amplifier), the sudden appearance of organic 



* Easily formed by placing a minute lens before the heliostat. 



t Dr. J. J. Woodward, U.S. Army Museum, has published a remarkable confirma- 

 tion of this point. The existence of intercostal beading, first discovered by the writer in 

 1862 and published in May 1869, has been (and still is) disputed by English observers 

 in no amicable spirit. He states: — ■ 



" Shortly after my paper had been forwarded I received a visit from Mr. Joseph 



2m2 



