Sept. 29, 1 881] 



NA TURE 



515 



but a tenth part of this, or a change of focus of the 

 lo,oooth of an inch, changes the appearance of the 

 magnified star-disk. These changes, so vivid and sudden, 

 produce a lively impression of the minuteness of the 

 wavelets of light which generate these diffractive pheno- 

 mena. 



The diameter of the brilliant disk, as miniatured, is 

 about the i-20,oooth of an inch; the jet black ring in 

 which it appears set, and indeed well set-off, is about 

 i-ioo,oooth thick. Slight changes in the focus, but espe- 

 cially slight changes in the corrections of the observing 

 and miniaturing glasses, produce a new order of pheno- 

 mena, full of a significant meaning and practical import. 

 The minimum visible is perpetually forcing itself upon 

 the observer's attention. The lightning-rod (here visible) 

 6000 inches away, is miniatured 600 times smaller than 

 at 10 inches by the 32nd objective, which then would 

 diminish an object only 320 times. The rod is therefore 

 depicted nearly 200,000 times smaller. It is exactly half 

 an inch in thickness ; its size therefore in the miniature 

 is I -400,000th. 



As the evening light faded away a long row of gas-lights 

 reaching half a mile came into view in pretty perspective, 

 the more distant being veiy slightly out of focus (six 

 divisions). The 32nd objective required to be advanced 

 the 4000th pirt of an inch for the distant light. In this 

 case the miniature was produced by a Zeiss i-32nd, and 

 viewed by another 32nd by the same maker. The 

 lowest eye-piece is employed and a shortened eye-tube. 

 A single glance at these microscopic landscapes satisfies 

 the observer at once as to the quality of the instruments 

 of observation. Achromatism is seldom attained without 

 generating a whitish haze, the inevitable accompaniment 

 of residuary spherical aberration. This haze is an in- 

 valuable indication. 



The haze observed in miniatures examined by high 

 magnifying power is an invaluable indication of sphe- 

 rical residuary aberration. The method gives a cruel 

 test of the optician's art. Its discovery led to the sub- 

 ject coming before the Royal Society and its being em- 

 bodied in their Transactions. When first seen it was 

 exhibited as a strong yellow fog. The announcement 

 of it occasioned the greatest surprise to the distinguished 

 makers of a "very fine" set for the writer. In most 

 cases the higher the angular aperture the denser was seen 

 the fog. The following is quoted : " Mechanical arrange- 

 ments are shown bv diagrams. Figs. I, Irf, plate LII." 

 \Pha. Tr., vol. ii. 1870, p. 592). 



" Expiiriiiicnt I. — Miniature of a small thermometer, the 

 ivory scale being graduated 24^ to the inch. -A. power of 

 300 diameters : low eyepiece 'A' and objective of one- 

 eighth focal length (made expressly for Podura testing) 

 was applied to view the miniature formed by a one-six- 

 teenth objective. The following appearances were care- 

 fully noted at the time of observation : — 



" Result. — The sparkle of light on the bulb of the 

 instrument, the graduation, and the mercurial thread 

 within the glass are invisible, obscured by a nebulous 

 yellow fog which no objective adjustments are able to 

 dissipate." 



In consequence of this unexpected discovery regarding 

 the quality of a " very fine " one-eighth, it was returned to 

 the opticians, to their surprise, for better compensation. 

 After improvements a very slight nebulous yellow cloud 

 now only remained. 



A new fact now came up. A miniature formed by an 

 imperfectly corrected glass is comparatively free from the 

 aberration shown by the same glass used as a microscope. 

 Thus, viewed by a good glass, the miniature of an in- 

 ferior one bore wonderful magnification by an excellent 

 objective. 



In innumerable objects surface markings are shown 

 only : with no perspective and with no foreshadow of 

 deeper structures such objects are opaque. But if trans- 



parent, the foreshadow of deeper structures confuses the 

 appearance of surface ; strange eidola are generated 

 difficult of interpretation and dispersion. .4 series of 

 star-disks in deeper foci intermingle their diffractions into 

 beautiful forms. 



The strong fact that these diffractions of a given disk 

 are wholly developed towards the eye of the observer, or 

 wholly developed beyond the true focal plane, according 

 as the correction of the glasses is over-wrought or under- 

 wrought, reveals an infallible clue to many spurious 

 effects. This method therefore more severely tests the 

 observing instrumentation than the miniature. 



The following experiment, arising out of the phenomena 

 in course of observation, is quite a microscopic study in 

 itself: — 



f.i'/tv/wcw/.— Miniature of garden view formed by a 

 i-3oth plano-convex lens and examined by a microscope 

 armed with i-8th giving 400 diameters. 



Result. — Landscape dark and hazy. But upon using 

 the same power (400) with a deep eyepiece and a half-inch 

 objective, there started forth an exquisite picture bril- 

 liantly lit up. Even the foliage glittering in the sunlight 

 was sharp, clear, and decisive, the details being marvel- 

 lously displayed. This large increase of light with 

 diminished observing angular aperture is at first sight 

 astonishing. 



It is to such causes doubtless that the microscopic world 

 so long disputed whether the markings on diatoms were 

 depressions or elevations. The earlier plates of such 

 objects, as given by Ouekett, teem with cidolic varieties 

 of form. It was thought impossible to resolve them, as 

 it was called, without complicated stops, which in fact 

 shut off the unsuspected residuary aberration- 



But it is necessary to pass on to the fundamental cir- 

 cular spectrum of a minute solar disk. A simple plano- 

 convex lens of half an inch focus is placed on the microsco- 

 pic stage and made to form a miniature of the prism star in 

 the field of the microscope. If the experiment be pro- 

 perly conducted, forty gorgeously coloured rings may be 

 counted. 1 have frequently examined these phenomena 

 with a power of 1000 diameters. Above the best focal 

 point is a bright fog ; below are seen the glorious diffrac- 

 tions. The sun, as far as it could be made out, was a 

 spurious disk nearly 6-ioo,oooths of an inch ; the first 

 black-ring, the blackest thing I ever beheld, was nearly 

 the i-5o,oooth. Each ring beyond appeared exactly of 

 the same breadth — nearly the I- 1 6,000th.' 



If now an over-corrected lens were substituted, the 

 diffraction rings ascended and the nebulosity descended : 

 thev exactlv ch.anged positions as regards focimetry. The 

 conclusion 'follows that all brilliant objects present dif- 

 fractions above or below the true focus according as the 

 observing instrument is over- or under-corrected by means 

 of the usual screw collar adjustment. 



In this wav the same object, especially diatoms, may 

 be made to take several very deceptive forms, because 

 spuriously diffracted. The same is true of all briUiantly 

 illuminated transparent structures. 



The intersection of an infinite number of cones of light 

 converging to different points of the axis here produces 

 the well-known interference extinction of undulation 

 evolving precisely-formed rings of darkness. The simple 

 lens, in these observations, develops circular spectra, as 

 being formed by an infinite number of prisms. 



.An important outcome of this phenomenon is the 

 unerring test (here presented by a simple single convex 

 plane lens) of the highest possible order, as to the quality 



' The contemplati in of these phenomena, utterly eclipsing in their brillUnl 

 beauty and precision of form the fainter diffraction phenomena described 

 bv Sir John Herschel as amongst the most gorgeous ni nature, aitonisDes 

 every beholder. Some little skill is required in gradually toning down the 

 excessive I might say painful, glories of the appearances— lengthening the 

 eye-tube ': glass tinted wedges (a single fieW-lens of a Hyghenian as eyepiece 

 produced a spectrum apparently twelve inches in diameter, measured with 

 the left eye by rule laid on the stage), or by camera. 



