MANAGEMENT OF THE MICROSCOPE. 165 



power/ cannot be obtained in the same combination; so that the choice 

 between two Objectives, one distinguished by the former of these attri- 

 butes, and the other by the latter, will depend upon the kind of work on 

 which it is to be employed. If the resolutions of the markings on Dia- 

 tom-valves is the Microscopist's special pursuit, 1 he will rightly prefer an 

 Objective of the largest attainable angle, with the best definition that 

 is compatible with it. But if he be engaged upon difficult Biological in- 

 vestigations, he will do well to make perfect i definition ' his sine qua 

 non, and to be content with the largest angle that can be obtained with- 

 out a sacrifice of this. It is, as already stated, in admitting of perfect 

 correction for Spherical Aberration, even to an aperture of 180, that 

 the great superiority of the ' immersion system ' consists; but the great- 

 est perfection in the construction of even an immersion Objective, 

 cannot (in the nature of things) prevent that impairment of defini- 

 tion, which has been experimentally as well as theoretically shown 

 by Dr. Eoyston Pigott to be consequent upon excessive widening of the 

 angle of aperture. The most serviceable Objectives for the most dif- 

 ficult Biological investigations, therefore, will (in the Author's judg- 

 ment) be such as possess the combination of qualities attributed by Mr. 

 Dallinger to the l-35th inch constructed specially for his work by Messrs. 

 Powell and Lealand; " the angle is moderate; its definition very crisp 

 and clear; and its penetration, considering its magnifying power, very 

 considerable." 



159. Test-Objects. It is usual to judge of the optical perfection of a 

 Microscope by its capacity for exhibiting certain objects, which are re- 

 garded as Tests of the merits of its Object-glasses; these tests being of 

 various degrees of difficulty, and that being accounted the best instru- 

 ment which shows the most ' difficult' of such tests. Now it must be 

 borne in mind that of the qualities which have been just enumerated, 

 the 'tests' usually'relied-on have reference almost exclusively to two 

 viz., definition and resolving power; and that the greater number of 

 them, being objects whose surface is marked by lines, striae, or dots, are 

 tests of resolving power, and thus of Angular aperture only. Hence, as 

 already shown, an Objective may resolve some very difficult test-objects, 

 and yet may be very unfit for ordinary use. Moreover, these ' difficult' 

 tests are only suitable to Object-glasses of very short focus and high mag- 

 nifying power: whereas the greater part of the real work of the Micro- 

 scope is done with Objectives of low and medium power: and the 

 enlargement of the Angular aperture, which enables one of these to re- 



1 It is assuredly neither the only nor yet the chief work of the Microscope (as 

 some appear to suppose) to resolve the markings on the siliceous valves of Dia- 

 toms ; in fact, the interest which attaches to observations of this class is entirely 

 confined to the value of these objects as ' tests ' of the performance of Objectives 

 ( 159). If one-tenth of the attention which has been devoted to the scrutiny of 

 these objects with instruments of the highest class, had been given to the study 

 of the Life-history of the minute Plants which furnish them, with such a Stu- 

 dent's microscope as thirty years ago enabled Mr. Thwaites to discover their 

 'conjugation,' it cannot be doubted that vast benefit would have accrued to Bio- 

 logical Science. It has been urged that the acquirement of the power of dis- 

 playing ' difficult ' Diatom-tests, is a valuable ' gymnastic ' for the training of Mi- 

 croscopists; but the experience of the Author, and of every Biological teacher he 

 knows, is that a much better training for the Student is to begin with the study 

 of such easy objects e. g., the Yeast-Plant, and Colorless Blood-Corpuscles, as 

 .afford him the experience which it is absolutely essential that he should acquire 

 in the first instance, and to proceed gradually from these to the more difficult, 

 gaining new knowledge at every stage. 



