184 Researches in Circular Solar Spectra. 



2. Various eye-pieces were inserted, and the lenses A, B, sepa- 

 rated or closed more or less till the most perfect definition was 

 attained. 



3. Now, regarding the instrument as a perfectly corrected com- 

 pensating eye-piece, it is transferred to the tube of another micro- 

 scope, the objective of which is again adjusted by the screw collar 

 for the most distinct definition. 



Tested on the principles glanced at in this paper, the corrected 

 compensating eye-piece, free from the usual aberrations, may be 

 confidently employed to test the circular solar spectrum of a minute 

 distant solar heliostat disk, formed in its focal plane, by the objec- 

 tive also of a proposed telescope. 



I trust observers with more convenient appliances and range of 

 prospect wiU be induced to try a series of experiments on correcting 

 residuary aberration in their object-glasses by the methods here 

 laid down, by whom the writer will at any time be honoured by 

 exhibiting some of the phenomena here described, as already done 

 to Dr. Gladstone, F.E.S., and Mr. Beaumont, F.K.S.* 



Appendix. 

 A. — Definition of Minute Organic Particles. 



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

 made for me by Messrs. Powell and Lealand ; with this glass, with- 

 out any obhquity, and using a tube 4 inches shorter than usual, and 

 a B eye-piece, the upper continuous ribs of the Podura were re- 

 solved 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. 



At present nothing is more difficult of definition in the micro- 

 scope 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, as far as is known to the writer, in displaying, in the 

 apparently blank spaces, 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 incht in dia- 

 meter appeared as large as a disk the sixty thousandth. To define 

 accurately bright organic particles, such as those of the smallest 



* And more recently to our Secretary, Mr. Slack, F.G.S. 



t Easily formed by placing a minute lens before the lieliostat. 



