178 COMPARATIVE VALUE OF OBJECT-GLASSES. 



parent objects by transmitted light. It is to these Objectives that 

 the remarks already made respecting Angular Aperture (§ 107, v.) 

 especially apply ; since it is in them that the greatest difference 

 exists between the ordinary requirements of the Scientific investi- 

 gator, and the special needs of those who devote themselves to the 

 particular classes of objects for which the greatest Resolving power 

 is required. A moderate amount of such power is essential to the 

 value of every Objective within the above-named range of foci : 

 thus, even a good Half-inch should enable the markings of the 

 larger scales of the Polyommatus argus (Azure-blue Butterfly) to 

 be well distinguished, these being of the same kind with those of 

 the Menelaus, but more delicate, and should clearly separate the 

 dots of the small or 'battledoor' scales (Fig. 335) of the same 

 insect, which, if unresolved, are seen as coarse longitudinal lines ; 

 a good 4-10ths inch should resolve the larger scales of the Pod lira 

 (Plate ii., fig. 2) without difficulty ; and a good l-4th orl-5th-inch 

 should bring out the markings on the smaller scales of the Podura, 

 and should resolve the markings on the Pleurosigma hippocampus 

 into longitudinal and oblique lines. Even the Half -inch or the 

 4-10ths inch may be made with angles of aperture sufficiently wide 

 to resolve the objects named as fair tests for the powers above 

 them ; but for the reasons already stated, the Author thinks it most 

 undesirable that they should thus be forced up to the work 

 altogether unsuited to their powers, by a sacrifice of those very 

 qualities which constitute their special value in the study of the 

 objects whereon they can be most appropriately and effectively 

 employed. And he is decidedly of opinion that an angular 

 aperture of 50° is as great as should be given to a Half -inch, 60° 

 to a 4-10ths inch, and 90° to a l-4th inch, that are destined for 

 the ordinary purposes of Scientific investigation ; whilst his own 

 experience would lead him to prefer an angle of 40° for the Half- 

 inch (§ 30), and of 75° for the l-4th inch, provided the correc- 

 tions are perfect.* Objectives fof these apertures should show 

 the easier Tests first enumerated with perfect Definition, a fair 

 amount of Penetrating power, and complete Flatness of field. No 

 single object is so useful as the Podura-scale for the purpose of 

 testing these qualities in a l-4th inch or l-5th inch Objective ; 

 and it may be safely said that a lens which brings out its markings 

 satisfactorily will suit the requirements of the ordinary working 

 Microscopist, although it may not resolve any but the easiest 

 Diatoms. In every case the Objective should be tried with the B 

 and C as well as with the A eye -piece; and the effect of this substi- 

 tution will be a fair test of its merits. "Where markings are undis- 

 tinguishable under a certain Objective merely because of their 



* The Author feels it due to Mr. Wheeler, who has specially applied 

 himself to carry out his views on this point, to state that the Half and 

 Quarter-inch constructed by him of the apertures above named are at 

 the same time of excellent quality and very moderate price. 



