1882.] 



MICROSCOPICAL JOUE^AL. 



173 



poses of original investigation, are of 

 great use for the demonstration of 

 certain classes of objects, to large 

 numbers at one time. 



" A certain Mr. Marshall appears 

 to have constructed the first com- 

 pound microscope, according to our 

 modern conception of the term. * 



* * Yet as late as 1821, we find 

 the great French philosopher, Biot, 

 insisting that 'opticians regard the 

 construction of a good achromatic 

 microscope as impossible,' and at the 

 same time Dr. Wollaston — the highest 

 authority upon this subject then in 

 England — gave it as his opinion ' that 

 the compound microscope would 

 never rival the single one,' * -" * 

 yet in less than two years thereafter 

 two French opticians, Selignes and 

 Chevalier, produced the redudio ad 

 absiirdum of all this a priori theoriz- 

 ing in the shape of compound achro- 

 matic objectives, consisting each of 

 two or more pairs of lenses, each 

 pair in turn, consisting of a double 

 convex glass ; and four years later 

 Amici produced an achromatic 

 combination, surpassing anything 

 previously constructed in this line ; 

 and from that time on, the principle 

 of combining two or more lenses, so 

 shaped and adjusted as to correct 

 each other's errors, was firmly estab- 

 lished. * * * To the discoveries 

 of Joseph Jackson Lister, and their 

 practical application by working op- 

 ticians, like Andrew Ross and Smith, 

 of London, we owe the production 

 of compound objectives of wide 

 aperture, flatness of field, and above 

 all, of highly perfected definition. * 



* * Then followed in rapid succes- 

 sion improvement upon improvement; 

 the immersion principle was utilized 

 (Amici and Hartnack) ; the aberra- 

 tion produced by the cover-glasses 

 corrected (Ross) ; the angle of aper- 

 ture increased to 135 degrees, which 

 for a long time was held to be 

 the largest attainable. Meanwhile, 

 there had grov/n up, in a little 

 village of this State, a young man of 



a scientific and practical turn of mind, 

 who had taken up for himself, and 

 by himself, the study of optics, and 

 had even in his boyhood, made with 

 his own hands, a microscope and 

 some telescopes, and later on had 

 done, though without much encour- 

 agement or patronage, good work as 

 a microscopist too. Reading Ross' 

 paper, and not feeling satisfied 

 with his theoretical reasoning, he soon 

 brought forth practical proof of the 

 correctness of his own instincts by 

 manufacturing a dry one-twelfth 

 inch objection of 146° aperture. 

 This young man was Charles A. 

 Spencer. The lenses which were 

 believed to have so nearly attained 

 the limit of perfection, fifteen years 

 ago (resolving Nobert's fifteenth 

 band, /. e. lines 1-9 1, 000th of an inch 

 apart) are antiquated now, and the 

 theoretical limit of perfection has 

 thus moved forward and forward like 

 the horizon, and seems destined ever 

 to recede. Thus Surgeon General 

 Woodward, of the navy, has since 

 resolved the entire nineteenth band 

 on the same plate." 



We have felt obliged to omit a con- 

 siderable portion of this address, 

 partly for want of space, and partly 

 because of a few minor errors of fact 

 which have crept in, and which should 

 be corrected before the address is 

 published in full. 



Mr. Henry Mills read a paper on 

 the fresh -water sponges. He review- 

 ed their classification by Carter, and 

 referred to the labors of Prof. Kelli- 

 cott, of Buffalo, and Mr. Potts, 

 of Philadelphia, in connection with. 

 American sponges. 



Mr. Ernst Gundlach presented a 

 paper on " Light and Illumination," 

 which seems to have stirred up quite 

 a discussion between some of the 

 members. Probably Dr. Gleason 

 summed up the question in a manner 

 satisfactory to all, when he stated it 

 as follows : " Do we see what we 

 see, or don't we see what we see, or 

 do we see what we don't see ? " It 

 seems no one can tell in every case yet. 



