Sept. 29, 1 881] 



NATURE 



5^7 



first intensely black first ring, fainter stars telescopically 

 show smaller disks. 



But whilst a dose row of spurious dis';s are seen to 

 coalesce and obliterate themselves if too close, and 

 become continuous as a thick luminous line — the neces- 

 sary effect of bright diffractions — duller objects devoid of 

 briUiance are feen of amazing minuteness of tracery. 



Exatnple. — The rungs or rounds of a ladder resting 

 against a house half a mile off were distinctly seen when 

 miniatured down to 1-1,000, 000th of their actual size, i.e. 

 considerably less than i-i,ooo,oooth of an inch. This feat 

 wTS accomplished by an immersion i-32nd by Seibert, 

 which diminishes an object 30,000 inches away just about 

 1,000,000 times. The bane of minute microscopic re- 

 search is thus seen to essentially consist of a combination 

 of diffraction with the haze of aberration. 



A blue glass evidently diminishes the diffrac- 

 tion phenomena ; so do neutral tints. This exactly 



tallies with the shrinking of spurious telescopic disks 

 during haze and sky-clouding. These facts forcibly 

 point out the great advantages of observing in mild' 

 light. In further support of this the writer has thus 

 effected several very difficult resolutions -in the " Ultima 

 Thule " of microscopic investigation glare is the prolific 

 parent of many fallacious interpretations. 



These studies have encouraged the writer to continue 

 a research into the limits of human microscopic vision. 

 In the case of bright illuminations the limit is evidently 

 reached at once. A minute refracting spherule thus 

 forms a bright focal point which itself exceeds by e.vpan- 

 sion into a spurious disk, the diameter of the spherule 

 producing it. Down to a certain size a focal image is 

 discernible. A very interesting study is given by the 

 solar star-disks presented by receiving the rays from the 

 heliostat after passing through a beetle's eye placed on 

 the field of view on the stage of the microscope. 



;. I.— Taking the objects fron. left to right, a represcntatijn is given of a miniature magnified a thousand times linear and the v.tri lus appearance; of the 

 heli.stat.c star disks with sl.ght changes of focussing, i Large diffraction rings: /ujuiamental spectrum given by a plane Convex len^ \ inch focal 

 length placed on stage of microscope. Forty rings have been ctunted. a. Optical gauge : various lenses showing spurious disks with minute diffrac- 

 tion rings similar to those on the *' bulls-t^ye " in centre of picture. 3. Intensely clear bright star disk produced by very perfect instrumentation. 

 4. A cross given by imperfect glasses. 5. Larger expanding rinis, the miniature or observing glasses being either under or overcjrrected. 6. The 

 finest and clearest spurious disk attainable. 

 Note — The house on the hill distinctly seen in this case of very finely corrected glasses. A miniature formed by a verj' fine i-32nd gives the distant 



ise and window nearly in the same focal plane. 



Until the sun shone the window appeared miniatured in 

 each eye. It seems curious to measure the focal length. 

 By measuring the images this was found to be i-ioooth 

 of an inch, giving enormous magnification for ordinary 

 vision.' The solar disk, however, appeared spuriously 

 enlarged. 



More wonderful diffraction-phenomena are developed 

 by different treatment. A half inch condenser-objective 

 was inserted between the coleopterous eyes and the 

 helioatat — behind or beyond the stage. The solar disks 

 developed then appeared severely beautiful. No such 



^ Their focal length was measured by selecting a well-defined object, as a 

 red brick house, carefully measuring micrometrically a given part of it, and 

 then measuring an image of the same thing in a known lens. 



If li be the distance ^f the object from its in age, »t the size of its minia 

 ture. M the size of the object. 



/= df.in^ M. 



A convenient formula for estimating the f cal length of a small lens was 



given by me in the Phil. Trans. If it is found to magnify ;« times at a 



distance between object and intake d, and if m be considerable, 



d rf 



/= — , more accurately := j 



wonderfully sharp black rings are even viewed tele- 

 scopically. These phenomena are in order of focal 

 classes — 



1. Intensely black truly formed rings. 



2. Hexagonal black patterns on a brilliant ground. 



3. Three such hexagonal rings to each eye-facet. 



4. Five such finished off with extremely rich Scotch 

 plaid patterns, highly coloured. 



G. W. ROVSTON-PlGOTT 



NOTES 

 The Delegates of the Clarendon Press will shortly publish an 

 " Elementary Treatise on Electricity," by the late Prof. James 

 Clerk Maxwell, edited by Mr. W. Garnett, formerly Fellow of 

 St. John's College, Cambridge. The book was commenced 

 about seven years ago, but its completion was prevented by the 

 author's other engagements ; S5 that during the last three years 

 of his life very little was added to the work. After his death 

 the fir-t portion of the manuscript, on Sta'ical Electricity, was 



