516 



NATURE 



\_Sept. 29, 1 88 1 



or inferiority of the observing microscope. Extreme 

 steadiness and a particularly delicate fine focussing 

 a Ijustment are indispensable for successful observations. 

 It is impossible here to detail them. 



A plain silvered mirror of the old style must be dis- 

 carded as a solar reflector; prismatic internal reflexion 

 or reflexion from a metallic surface is indispensable for 

 producing purity of spectra.'' 



But it is when superb objectives are placed as it were 

 eye to eye, that the finest observations can be made 

 (especially when both are used with their noses inserted 

 in the proper immersion fluid for observing landscapes) 

 on distant objects. 



The extreme delicacy of the change in focal planes of 

 vision is charmingly illustrated by observing with a good 

 i-8th the miniature of two brilliant points, the one 200 

 inches, the other 201 inches away in the same line. It 

 the miniature is formed by a simple lens, theoretically 

 the focal images of these star-disks would be separated 

 by an interval of 2-io,ooo,oooths of an inch for a i-ioth 

 lens, perfectly aplanatic." 



These disks, by their rapid change in appearance, give 

 the most exquisite means of determining focal changes 

 in the microscope. Thus for a focal depression of 

 7-io,oooths of an inch a change took place from one 

 pure jet black diffraction-ring round central disk to eleven 

 rings. The rings changed visibly for a focal depression 

 o: 100,000th of an inch. 



The greatest confusion has existed regarding the terms 

 penetration, definition, and resolution. The study of star 

 disks miniatured by surpassingly well-corrected glasses 

 furnishes the observer with a new order of facts upon 

 which irrefragable conclusions can be founded. 



A telescope of very fine quality should have a focus of 

 extreme delicacy. C ne whose change of focus by i-ioth 

 of an inch produces little or no effect upon the " defini- 

 tion "is contemptible. In the same manner the quality 

 oi- microscopes may be estimated by the striking effects 

 of a minute focal change. The planes of focal vision, it 

 will be found, vary extremely, their interval varying 

 according to a function of several complex factors. 



The prism-heliostat already described is well repre- 

 sented by an Amici prism. Small bulls-eye lenses, 

 laid with the convex surface upwards in the sunshine, 

 present two brilliant images of the sun at a distance of 

 200 inches. They are of unequal brilliancy. A row of 

 these placed in a line with the axis of the instrument,' and 

 somewhat tilted, so that the star-disks may be all seen at 

 once, will develop a series of fine microscopic effects, 

 dependent on the corrections of the systems in use and the 

 immersion-fluid in which the noses of the glasses are in- 

 serted (sometimes apiece of adherent glass cover interven- 

 ing', as the water tends to run off). If a i-33rd objective be 

 employed to miniature them, and a i- i6th to observe them, 

 extraordinary fine excellence insures a perspective almost 

 as clear as an opera-glass. The minute double stars — which 

 may be brought as close as we like by change of the angle 

 of position of the instrument with the sun's azimuth — 

 produce a variety of diffraction rings, mingling, crossing, 

 and breaking up each other in a manner that could hardly 

 be suspected during the telescopic observation of real 

 double stars. Seldom can a bright landscape be attained 

 without leaving some little outstanding colour. If that 

 be destroyed by changing the glasses or corrections im- 



' Some indications of quality in microscopes; — (i) Confused mass of spurious 

 d.^ks oddly arranged ■ (2) Beauty of rings utterly marred ; (3) Very few rings 

 definable; {4) Spectrum notched, grained, and spotted; (5) Systems of 

 excentric rings dark and coloured, much confused; (6) An ''engine-turned 

 pattern " mottled and degenerated ; (7) Achromatism and freedom from 

 sp'ierical aberration in all cases found incompatible. The universal presence 

 of same residuary spherical aberration is demonstrated by several irrefragable 

 pro jfs in all the finest-made modern gla!.ses. 



^ First conjugate focus distance o'iooo5oo2 



Second ,, „ o'iooo4g77 



Interval between foci ... o"oooooo25 



3 The miniature.making objective and observing microscope arc both 

 plated horizontally. 



mediately, all black objects look grey, and the grey becomes 

 lighter as the colour nearly vanishes The rest of the 

 view is charged with a thin greyish-white nebulosity, the 

 sure indication — as already described — of residuary aber- 

 ration. Just within the best focus an exquisite little 

 dark-grey bead margined with black — much le^s than the 

 primary brilliant disk (half its size) — may be discovered 

 by close attention and a particularly well-adjusted focuss- 

 ing apparatus. 



In observing these interesting phenomena it will be 

 seen that larger' and smaller spurious disks are formed 

 according to the curvatures of the bulls-eyes ; but there 

 is one minimum size ; and these, when contiguous, illus- 

 trate diffraction spectra in a brilliantly instructive form.^ 



Upon a proper arrangement, putting the bulls-eye and 

 the instrument nearly in a line with the sun's azimuth, a 

 superb representation of the double star, Castor, is seen, 

 the fainter star being that caused by internal reflection. 

 Intensely black diffraction rings round each, and several 

 fainter ones, fewer as the quaUty of instrumentation is 

 raised. Perfect roundness can only be attained by exact 

 coincidence of the optical a.xes of the system. Very 

 slight obliquity (even half a degree) causes the rings to 

 overlap and bulge on one side. Much obliquity gives 

 rise to glorious curves of the three orders of the conic 

 sections, of wondrous beauty and precision in effulgent 

 colours. 



Mercurial globules near the microscope e.xhibit very 

 delicate and complex forms when similarly miniatured, as 

 minute solar disks, in sunhght. 



Experiment. — An optician's gauge comprising half a 

 dozen lenses of standard foci i" to i-6"th, lying in the 

 sunshine, miniatured star disks by reflection (see (i) in 

 figure. Inferior objective \\ inch examined with fine 

 power of 1000. Two brilliant crimson disks in contact 

 expanding within focus to an oval ring of deep crimson 

 beads. 



Experiment. — If the image of the sun be received on 

 white paper from a small lens placed at various degrees of 

 obliquity peculiarly beautiful forms are seen fringed with 

 colour. When the lens is sufficiently minute these spectra 

 exhibit tothemicroscope exquisitely-arranged curves in jet- 

 black lines ; circular elliptic parabolic and hyperbolic, with 

 inexhaustible variety, according to the focal plane of 

 vision and obliquity. Heliostatic star-disks inost success- 

 fully exhibit these unique phenomena. The superiority of 

 these phenomena to anything telescopic of the sort is 

 insured by the absence of atmospheric disturbance within 

 so short a distance. They are all under instantaneous 

 control. 



The limits of human vision among so many bright 

 points are patent enough. So long as there is bright sun- 

 shine every glittering point obscures, I might say utterly 

 effaces, the finer traceries of detail. A pasring cloud, 

 however, brings them all out with astonishing fidelity. 

 Brilliant diffraction is thus demonstrated to be incoiii- 

 patible with exact portraiture. The limit is reached in 

 brilliant sunshine by the diffraction disks obliterating the 

 very objects which produce them. This limit is well 

 measured by the diameter of the smaller disks seen in 

 contact, which in white compound light generally appears 

 by micrometric measurement to be between the i-8o,oooth 

 and i2o,oooth of an inch in the microscopic field. 



We need not be surprised at this variation : the undu- 

 latory theory of light gives one size only. Yet, as the 

 spurious disk by theory is shaded off gradually into the 



* Luckily the image of the sun is very nearly the hundredth part of the 

 focal length of the lens employed. If a bulls-eye of i-inch focal length 

 be employed at 200 inches, and a miniature be produced by a i-8th, dimin- 

 ishing it 20 X 80, or i6co times, the observed im.ige of the sun would theoretl- 

 c-illy be 1-1600 X 100 = 11, 600,000 less than one millionth of an inch. Il 

 should be n ted that if the primary axial image of the sun be too large, _rt« 

 diffraction spectra will be developed at 200 inches, unless very deep minia- 

 ture-objectives be employed, . , , . ,^ 

 = The Ianu.iry sun image formed by i-inch lens ic i-io6th inch. 



The April „ „ .. i-io8th inch. 



The July „ ., >, i-iogth inch. 



