312 



THE POPULAR EDUCATOR. 



that he lays great stress very properly on the use of a rectan- 

 gular glass prism as one of the best illuminators ; and it is 

 strange that philosophers and practical men did not long ago 

 move on a little further in their modification of the prism 

 system of illumination. 



Tho credit of the proper use of the reflecting prism, however, 

 belongs to the Eev. J. B. Eeade, of Bishopsbourne Eectory, the 

 President of the Eoyal Micro- 

 scopical Society, who announced 

 his discovery in the following 

 letter addressed to Mr. Samuel 

 Highley : 



" I have jftst hit upon another 

 and different mode of illumina- 

 tion, cheaper certainly than the 

 kettledrum shall I also say 

 better ? I obtain, at all events, 

 such an amount of light that 

 with the 3^ object-glass I can use 

 an eye-piece double in power to 

 the D eye-piece. I thus get at 

 the knowledge of the exact struc- 

 ture of the markings of P. angu- 

 latum,* which are, in fact, a 

 series of siliceous hemispheres 

 arranged in beautiful order on 

 the plane of the valve, thus (as 

 shown in Fig. 4a). Yours in haste, 

 and very truly, J. B. EEADE. 



"May 28, 1869. A power 

 shows the real structure of P. 

 .formosum splendidly. It is liko 

 a field of haycocks. So that the 

 question about elevations and 

 -depressions is set at rest. 



" J. B. EEADE." 



The markings thus developed 

 by Mr. Eeade' s mode of illumina- 

 tion will remind the reader very 

 much of the different appearance 

 presented by certain nebulae, 

 when viewed by telescopes of 

 high, and those of a still higher 

 power. 



The late eloquent and learned 

 astronomer, Dr. Nichol, speaking 



of the nebulae, says : " Turning to the dimmer objects, those 

 which heretofore half known and half only conjectured of 

 have rested on the very verge of the sphere of observation, it 

 is there that wo have chiefly been astonished by the feats of the 

 new instruments. The Dumb Bell Nebula is known all over the 

 world by the excellent drawing of it by Sir John Herschel, 

 which I have attempted to reproduce ia Fig. 6. Look at it 

 in Fig. 7, where it is as I 

 have seen it with Lord Eosse's 

 three-feet mirror no longer 

 distinctness or completion of 

 form, but a strange mass, 

 internally most irregular, 

 clustering apparently around 

 two principal nuclei or knots 

 of stars,f and presenting, when 

 it merges into the dark, the 

 utmost indefinitensss of out- 

 line." 



Eeturning to the new mode 

 of illumination devised by M- . 

 Eeade, let the inventor sneak 

 for himself. He enthusiastically says (genius always provokes 

 this weakness) 



" A single sentence will be sufficient to describe the diatom- 



* Pleuroirigma angulatum. 



t Since carefully examined by William Hugging, Esq., F.E.S. The 

 spectrum of this nebula consists of one line only, N. A prismatic 

 examination of the light from different parts of this object showed 

 that it is throughout of a similar constitution, and ia probably gaseous, 

 and does not consist of a cluster of stars. See Fig. 5 on opposite 

 page. 



prism illumination. I place an equilateral prism (Fig. 8) below 

 the stage of the microscope, and the light, either of the sun or of 

 a lamp, after being totally reflected, is made to fall obliquely on 

 the valve to be examined. The light of a lamp is condensed in 

 parallel rays by means of a bull's-eye lens. This is all. But 

 why never used ? Is it possible that, without making the trial, 

 a supposed deficiency of the power of a few parallel rays could 

 prove a bar to the experiment ? 

 Yet it would almost seem as if 

 such were the case, since Newton, 

 Chevalier, Amici, Brewster, and 

 Abraham have suggested different 

 modes of obtaining condensed and 

 convergent reflected light, and 

 their prisms have frequently 

 formed adjuncts for microscopical 

 examination. But, be this as it 

 may, the fact remains that we are 

 still without any authoritative 

 recommendation to adopt the 

 method I have described. Its 

 advantages, however, are great 

 and obvious. I have no longer 

 two suns in my firmament shining 

 at right angles to each other, but 

 one source of proper light pro- 

 perly placed ; and, therefore, 

 instead of the false appearance 

 of lines and striae, rectilineal and 

 oblique, unde'r low powers, and of 

 hexagons and other fancies, under 

 high powers, I see what really 

 does exist, viz., a series of beauti- 

 ful hemispheres placed in their 

 due order on the siliceous tissue 

 of the valve. The kettledrum 

 with its double pencil of light is, 

 therefore, quoad hoc, a thing of 

 the past. If the hemisphere on 

 the stage were really the size 

 which our powers make it, nearly 

 half an inch in diameter, it would 

 be seen by unassisted vision, and 

 we should smile at a supposed 

 necessity of forming its shadow 

 by two sources of light, just aa 



an artist would smile if he were advised to have two windows 

 in his studio at right angles to aoh other, for the more artistic 

 illumination of his sitter. The ruoun, as shown by the sun's 

 illumination, is a fair illustration of diatom-illumination. Light, 

 virtually parallel, falliTig obliquely on one side only of its moun- 

 tains and craters, produces naturnl light and shade. Any other 

 arrangement would fail, and for this reason right-angled aper- 

 tures, either with the kettle- 

 drum or the prism, lead to 

 illusions. The kettledrum, how- 

 ever, with one aperture properly 

 placed, ia still a serviceable 

 condenser, and brings out the 

 hemispheres remarkably well. 

 Still, refracted light has not 

 the power and purity of re- 

 flected light ; and converging 

 rays, whether reflected from a 

 convex prism or refracted 

 through a lens, must yield the 

 palm to parallel light, which 

 is obtained by Newton's plane 

 prism as from the sun. The truth of this remark will be obvious 

 if we place the smaller hemisphere of the kettledrum at right 

 angles to its present position, and use it for obtaining condensed, 

 reflected, and convergent rays from its flat surface, as proposed 

 by Brewster. In this case the object, being in a cone of con- 

 verging rays, is virtually under the influence of more than one 

 source of light, and its character is lost amid thp Intense illu- 

 mination. It would be easy, by means of a double concave lens 

 placed within the focus of the converging cone, to produce an 

 intense beam of parallel light without any assistance from the 



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