1881.] 



MICROSCOPICAL JOURNAL. 



173 



some advice, such as Dr, Carpenter 

 has given, will prove of interest. 



For the general purposes of the 

 microscopist, Dr. Carpenter strongly 

 favors moderate angular apertures. 

 He says, referring to the wide angles : 

 " But here comes in another source 

 of impairment [of definition], — the 

 difference in the perspective views of 

 every object not a mere mathemati- 

 cal point or line, which are received 

 through the different parts of the 

 area of the objective," and as this 

 difference " increases with the angle 

 of aperture of the objective, its defin- 

 ing power must be proportionally im- 

 paired." If this is intended to imply 

 that increase of angular aperture im- 

 pairs definition, then we cannot agree 

 with the writer. But we are inclined 

 to put a different interpretation upon 

 the results to which he alludes in 

 support of the above statements, and 

 to say that any apparent defect in 

 the definition of a good wide angled 

 glass, in a particular case, is due, not 

 to imperfections of the objective, but 

 to the unsuitable nature of the ob- 

 ject, for examination with that glass. 

 The experiments alluded to by Dr. 

 Carpenter do, indeed, tend to show 

 that two or more objectives of different 

 apertures, may be used with advant- 

 age in the study of unknown objects ; 

 for the appearances of the image may 

 be materially altered by a change of 

 the angular aperture. But apart from 

 the question of definition, it must be 

 conceded that for general work, the 

 moderate apertures possess qualities 

 which make them to be preferred 

 above all others. Dr. Carpenter states 

 that " it is also the experience of 

 Messrs. Dallinger and Drysdale, that 

 for the definition of the immeasurably 

 minute reproductive granules of the 

 Monadine forms, * * * * or of the 

 flagella of Bacterium termo, which 

 may be characterized as the highest 

 feats of biological microscopy yet 

 performed, moderate angles of aper- 

 ture are unquestionably to be pre- 

 ferred." We were somewhat surprised 

 to read this passage, but we have no 



reason to question its truthfulness, 

 although at first, we would have 

 supposed that for such work, the 

 widest angles would be the best. 

 But the resolution of close lines, and 

 the definition of isolated particles, or 

 flagella, do not require the same opti- 

 cal qualities. This fact has been too 

 often overlooked in controversy. 

 In the face of these statements, and of 

 the testimony of Prof. E. Abbe, who 

 himself recommends the use of mo- 

 derate apertures, and, we may add, 

 of the almost universal verdict of ex- 

 perienced microscopical investigators 

 in all countries, we fail to see why a 

 few persons should persist in decla- 

 rations contrary to these. The trouble 

 has been that objectives have been 

 studied and judgement passed upon 

 them from a point of view entirely 

 too narrow. It is one thing to try a 

 lens upon test-diatoms, and care- 

 fully prepared histological specimens, 

 and quite another matter to use it in 

 the daily work of examination, when 

 it would be a useless and unnecessary 

 waste of time to carefully mount 

 every specimen. 



We have, therefore, the concurrent 

 testimony of those men, who are un- 

 questionably able to speak with au- 

 thority on the subject, that " great 

 revolving power is only exceptionally 

 needed in the most difficult biological 

 investigations ; what is especially- 

 required for the study of living and 

 moving organisms, being such a crisp 

 and clear definition, good working- 

 distance and considerable local depth, 

 as high-power objectives of the widest 

 aperture cannot afford." 



The fallacy of judging the quality 

 of an objective by subjecting it to 

 any one test, has been recognized by 

 the practical experience of a gentle- 

 man in this city, who has given con- 

 siderable attention to the comparison 

 of objectives. As the results of his 

 examination of three lenses by diffe- 

 rent makers usmgA.pellucida as a test 

 object, he considered that there was 

 very little choice between them, al- 

 though there was considerable diffe- 



