654 



MICROSCOPE. 



of both lenses and specula. We refer tlie reader 

 to our former article for sufficient notices of these 

 various constructions, as it is not intended in this 

 place to enter into extended descriptions of instru- 

 ments that have been long in use, and which are 

 consequently well known ; our present purpose is 

 to speak of those improvements in the elements 

 of the microscope that have opened up visions of 

 wonder in the world of atoms. See Catoptrics, 

 Dioptrics, former article Microscope, Optics, &c. 



From the observation of Seneca, (see former 

 article) we find that the ancients had some notion 

 of microscopic power, and that globes of water of 

 different diameters were the magnifiers in use 

 amongst them. Since the sphere therefore seems 

 to connect itself with the origin of the microscope, 

 it may be an orderly inquiry to trace the varieties of 

 the spherical figure that came successively into use, 

 and to state the ingenious expedients by which re- 

 cent philosophers and men of science have suc- 

 ceeded in improving its performance as a magnify- 

 ing power. 



SPHERES. The discoveries made by the ancients 

 with their spheres were few and unimportant, and 

 indeed they could not well have been otherwise ; 

 for in addition to the disadvantages resulting from 

 the low refractive power of water, we presume 

 they bad no certain means of producing a truly 

 spherical figure, and consequently they would be 

 greatly perplexed by the eccentric refractions of 

 their magnifiers, and the illusive distortions thence 

 resulting. The glass spherule, or minute sphere, 

 obtained considerable reputation in the infancy of 

 the microscope, and indeed maintained it for some 

 time, in consequence of its high magnifying power, 

 and the facility with which it could be formed to 

 almost any degree of minuteness ; and as the focal 

 distance of any glass sphere or spherule is distant 

 from its surface, only the one-fourth of its diam- 

 eter, a spherule of exceedingly small radius natu- 

 rally appeared to offer the means of enlarging the 

 angle of vision to its greatest possible extent, or, 

 in other words, of reducing the seeing distance be- 

 tween the eye and an object almost to a physical 

 point. It was soon however discovered that these 

 deep magnifiers were imperfect in performance, 

 owing to the small quantity of light they trans- 

 mitted to the eye, and the contracted field of view 

 consequent on their limited aperture, and more 

 especially on the very unequal refraction of the 

 oblique rays produced by their spherical figure. 

 They maintained their repute notwithstanding, for 

 by their assistance the philosophers of the age were 

 enabled to look into the hidden things of which 

 philosophy had never dreamed ; and men were con- 

 tent to strain their optic nerves and tolerate other 

 inconveniences, in return for the advances they 

 were daily enabled to make into a world of know- 

 ledge hitherto untracked. When the novelty of 

 these new and interesting observations had in some 

 measure passed, the spherule gradually fell into 

 disuse ; and magnifiers of less power, but admitting 

 more light, and presenting a larger field of view, 

 were substituted in its place. It is not however 

 to be understood that the spherule has ever been 

 entirely laid aside ; for even at the period when no 

 other method had been suggested for making it 

 than that of melting a thread of glass in the flame 

 of a candle, and when there was consequently to 

 be added to its other inconveniences the formidable 

 one of imperfect sphericity, many persons strongly 

 attached to deep powers, and disposed to consider 



amplification of the image us the highest achieve- 

 ment ol the microscope, held an opinion that no 

 good instrument could dispense with its assistance. 

 Afterwards, when opticians had succeeded in yrind- 

 iny minute spheres of perfectly true figure, they 

 came once more into somewhat general use; for in 

 despite of all their disadvantages, they did show a 

 physical point of an object distinctly and highly 

 magnified, and this was enough for the mass of ob- 

 servers. Indeed, even in the present day, these 

 spherules are included in the catalogues of our op- 

 ticians, and are offered to the curious observer, of 

 so deep a focus as one 300th of an inch. All we 

 need say of them is, that they are the very worst 

 class of magnifiers in use at this time. We now 

 come to speak of the recent ingenious contriv- 

 ances for rendering the sphere a satisfactory magni- 

 fying power. 



The Grooved Sphere. Sir David Brewster, it 

 would appear, originated this device, and that 

 being the case, we are at a loss to conceive why a 

 slight modification of it should have obtained the 

 name of the Codding ton Lens; for the latter ap- 

 pears to differ from the grooved sphere only in its 

 being made more minute, and having the central 

 thickness diminished. The grooved sphere is no- 

 thing more than a sphere with a concave groove or 

 hollow, more or less deep, cut round it. The end 

 obtained is a larger available field of view, and an 

 image more clearly defined, in consequence of the 

 extremely divergent rays being cut off and pre- 

 vented from entering the eye. 



A B C D is a sphere having 

 the parts A E C, B F D cut 

 away entirely round it; ex- 

 tremely divergent ra\s falling 

 upon the surface A B, or C 

 D, would consequently never 

 reach the eye; and by dimin- 

 ishing the central thickness 

 E F, the pencils of rays might 

 be confined so closely within the vicinity of the centre 

 of the sphere, as to reduce the spherical and 

 chromatic aberrations to nothing, in a practical 

 view. 



We have a grooved sphere of one-tenth of an 

 inch focus, made by Mr Gary of the Strand, which 

 defines the object beautifully; for an illustration 

 of its penetrating power we refer to fig. 32, pi. 

 90, representing all the interesting details in the 

 tongue or trunk of a fly. 



The Periscopic Sphere. This is another con- 

 trivance by Sir D. Brewster, though he appears to 

 have been led towards it by the periscopic doublet 

 of Dr Wollaston. However this be, he has greatly 

 improved upon the original hint, and produced a 

 sphere which obviates all the inconvenience arising 

 from diminution of light by duplication of surface, 

 and the confusion resulting from spherical aberra- 

 tion. From the several methods of construction, 

 we select the one exhibited in our diagram. 



A eg C, B/A D, are two 

 perfectly similar double con- 

 vex lenses, united by a small 

 tube e f g h, in such a way 

 that their sides A C, B D, 

 may coincide with the surface 

 r , of the sphere A B C D, of 

 which indeed they form parts. 

 The tube e fg h, connecting 

 the centres of the lenses, is to be filled with a 

 fluid, having the same refractive power as they 



