168 THE MICROSCOPE AND ITS REVELATIONS. 



purposes, with an aperture not exceeding 80. Larger-angled l-4ths and 

 l-5ths cannot be conveniently used for opaque objects, unless these are 

 shown by Prof . Smith's or some analogous illumination ( 116). The 

 great value of these powers lies in the information they enable us to 

 obtain regarding the details of organized structures and of living actions, 

 by the examination of properly-prepared transparent objects by trans- 

 mitted light; and it is to them that the remarks already made respecting 

 Angular aperture ( 158, n.) especially apply; since it is here that the 

 greatest difference exists between the ordinary requirements of the 

 Scientific investigator, and the special needs of those who devote them- 

 selves to the particular classes of objects for which the greatest ( resolv- 

 ing' 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 dis- 

 tinguished 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. 416) 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 Podura (Plate II., fig. 2) without difficulty; and 

 a good l-4th or l-5th-inch should bring out the markings on the smaller 

 scales of the Podura, and should resolve the markings on the Pleuro- 

 siyma angulatum into lozenge-lines, the B and eye-pieces being used 

 when the scales are very small and their markings delicate. Even the 

 half-inch or the 4-10ths inch may be made with angles of aperture suffi- 

 ciently wide to resolve the objects named as difficult tests for the powers 

 above them; 1 but for the reasons already stated, the Author thinks it 

 most undesirable that they should be thus 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-ftich, 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 ( 39), and of 80 for the l-4th inch, provided the correc- 

 tions are perfect. Objectives of these apertures should show the easier 

 tests first enumerated, with perfect Definition, a fair amount of Pene- 

 trating 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 



1 By Mr. Tolles (Boston, N.E.) the Angle of the half-inch is carried to 80; and 

 that of the 4-10ths to 145. And it has lately been seriously maintained that an 

 Objective of the latter focus supplies almost every need of the Biologist, since, as 

 even difficult Diatom-tests can be shown by it, it can be worked up by deep Eye- 

 piecing to the highest power that he requires, except for special investigations. 

 But the resolution of a Diatom is one thing, while the prosecution of investiga- 

 tion continued through several hours at a time is quite another; and the Author, 

 regarding the advice of this writer as most dangerous to the eyes of those who 

 may follow it, deems it his duty to enter his protest against it. Many excellent 

 makers now make first-class Objectives of narrow as well as wide angles; thus, 

 Messrs. Powell and Lealand, followed by several others, make the half-inch of 

 40 (first constructed for the Author, to be used with the Stereoscopic Binocular), 

 as well as a half -inch of 70; Messrs. Beck make a 4-10ths of 55, as well as one of 

 90; and Mr. Crouch a l-4th of 60, another of 105, and another of 140. 



