58 



HARD WICKE 'S S CIENCE - G OS SI P. 



and " aplanatic searcher " will convince the impartial 

 observer that in principle and in construction they are 

 opposed to geometrical and practical optics, and break 

 down on being put to experimental proof. 



Not many years since similar attempts were made 

 to interpose an "amplifier" between the object and 

 field-glass of the telescope ; but it led to no practical 

 results. Barlow's lens, an elegant optical toy, is not 

 known to have been the means of making any dis- 

 covery in the hands of the astronomer ; on the con- 

 trary, it was, soon after ^its introduction, discarded 

 because it produced spectral images. That any mere 

 amplifying apparatus should have been seriously pro- 

 posed for obtaining "transcendental definition" and 

 increasing the power of observers with the microscope, 

 is very sm-prising. It is asserted by the inventor of 

 the " aplanatic searcher " that in principle the instiii- 

 ment involves the correction of the residuary chro- 

 matic and spherical aberrations in the image-pencil 

 after it leaves the objective, and is in course of pro- 

 jection to the conjugate focus. This, it will be 

 observed, is equivalent to asserting that every lens 

 made is, to some extent, imperfectly constructed ; and 

 this I suppose no practical optician will dispute. 

 But if this i-easoning be followed out to its logical 

 conclusion, we are brought to the strange and almost 

 inconceivable proposition that an objective constructed 

 as nearly as possible on the most rigid geometrical 

 formula;, but with a residuum of uncorrected aber- 

 rations, all its errors are made to vanish by means of 

 the "aplanatic searcher." This contravening accessory 

 to the microscope has been conscientiously employed 

 by a number of practical men ; among others, a former 

 President of the Royal Microscopical Society of 

 London. He writes of it, ' ' Even the name of the 

 instrument, a ' searcher,' seems to imply that its 

 application is wholly empirical ; even in his own 

 hands it has been observed that while the desired 

 appearance is sometimes speedily produced, at other 

 times a considerable amount of manipulation has 

 seemed to be required for that purpose. Under these 

 circumstances it appears that the controversy that has 

 existed as to the beaded appearance of podura scales 

 must still be considered an open question. Reviewing 

 this, however, under the dictates of common sense, 

 when observing the familiar podura ' notes of admi- 

 ration,' well defined and free from colour, I cannot 

 resist the inference that in the objective all aberrations 

 are nicely balanced, and the object truly represented 

 ' in the visual image ; on the contrary, when the same 

 object is viewed as rows of ill-defined beads loaded 

 with colours, it is difficult to avoid suspecting that the 

 appearance is a spectral illusion, resulting from some 

 unexplained diffraction or interference ; and this sus- 

 picion can hardly be dispelled by anything short of 

 rigid mathematical demonstration."* 



* Charles Brook, P'.R.S. President's Address. Monthly 

 Microscopical Journal, February, 1874, p. 94. 



Admitting then, that the " aplanatic searcher" 

 does "amplify magnifying power," it is mere 

 amplification, that can be obtained by various well- 

 known eye-pieces, tried, but ^not even generally 

 accepted, by microscopists. That any piece of 

 accessory apparatus of the kind, ' ' searcher " or 

 "amplifier," should be vaunted as "an improve- 

 ment," or as a valuable means of "increasing the 

 resolving or defining power " of the instrument, at 

 this advanced age of its history, is to me quite 

 remarkable. That the makers of such accessory 

 appliances have never ventured to exhibit them in 

 public, is also very significant ; it is reasonable to 

 suppose that, had they possessed any merit, they 

 would have been the first to bring them prominently 

 into notice, and advocate their employment with a 

 zeal quite equal to that of their inventors. This, 

 however, has not been done ; and without entering 

 deeply into the optical principles involved in the 

 construction of the "' aplanatic searcher " — and which 

 would involve a fruitless discussion on empirical 

 optics,— it will occur to every practical mind, that, ■ 

 supposing such an accessoiy to effect some amount of 

 correction— and the omis of proof or demonstra- 

 tion rests with its inventor, — it can be unmistakably 

 shown that residuary aberrations should be removed 

 by a more rigid adherence to mathematical formula; 

 in the construction of the objective, which would 

 involve far less expenditure of optical means, and 

 loss of light, than by any extraneous arrangement of 

 lenses in the body of the microscope. 



In what way " aplanatic searchers and amplifiers " 

 tend to increase errors of interpretation rather than 

 the elucidation of structure, is intelligibly explained 

 in Dr. Woodward's valuable communication to the 

 Mo7ithly Microscopical Journal, " On the Structure of 

 the Gnat's Body-scale."* " By varying the illumina- 

 tion from a strictly central to a gradually increased 

 oblique condition, all the more familiar phenomena 

 of diffraction can be produced, even to the beaded 

 structure." Microscopists are well acquainted with 

 the fact (and it is almost unnecessary to recall it to 

 the minds of the members of a learned society), that 

 if we interpose minute diaphragms between a strong 

 source of light and an objective, the phenomena of 

 diffraction will be produced. Such objects as gnats' 

 body-scales, podura scales, or the frustules of diato- 

 macere, may be regarded as so many minute forms of 

 diaphragms, not necessarily diaphragms in the com- 

 mon acceptation of the term, that is, apertures cut 

 out of an opaque material ; but in them we have 

 tissue or substance more or less transparent and re- 

 fractive in combination, in the highest degree com- 

 plex, and producing the phenomena of diffraction in 

 almost infinite variety and complexity. The admir- 

 able photographs of the gnat's body-scale, made by 

 Dr. Woodward, conclusively prove this, and as effec- 



* Monthly Alicroscopicai yournal, vol. xv. p. 253. 



