MANAGEMENT OF THE MICROSCOPE. 



171 



Direction Striae, in 1-lQOth of an inch* 

 of Strice. SMITH. SOLLHT. 



Good specimens of the first ten of the foregoing list may be resolved, 

 with judicious management, by good small-angled l-4th or l-5th inch 

 Objectives, and even, with very oblique illumination, by Objectives of 

 one-half and 4-10ths inch, having an angular aperture of 90; the re- 

 mainder require the larger aperture proper to the l-8th inch or higher 

 power, for the satisfactory exhibition of their markings. The first column 

 of measurements in the above table gives the numbers stated by Prof. W. 

 Smith as averages; the second column gives the numbers subsequently 

 assigned as the extremes by Mr. Sollitt, 1 who pointed out that great dif- 

 ferences exist in the fineness of the markings of specimens of the same 

 species obtained from different localities a statement now so abundantly 

 confirmed, as to be entitled to rank as an established fact. It is in regard 

 to Amphipleura pellucida, however, that the greatest diversity of opinion 

 has existed; and the conclusion which the Author had expressed in the 

 earlier editions of this Manual, that Mr. Sollitt's estimate was much too 

 high (having been based on ' spurious ' lineation), has been fully con- 

 firmed by Col. Dr. Woodward; who, having succeeded in obtaining very 

 perfect Photographs of this Diatom, under powers of 1500 and 1650 diam- 

 eters, has found that the striae on the largest valves were never more than 

 91 in l-1000th of an inch, while those on the smallest never exceeded 100 

 in the 1000th inch. 2 The ' resolution' of the lines on this test may be 

 made without much difficulty by ' immersion' Objectives of l-8th inch 

 without any excessive Aperture; but the resolution of the lines into dis- 

 tinct dots is a severe test for Objectives of largest Aperture. Several 

 very difficult tests of this description have been furnished by the late 

 Prof. Bailey 3 of West Point (U.S.); among them the very beautiful Gram- 

 matophora subtilissima and the Hyalodiscus subtilis, the latter being of 

 discoid form, and having markings which radiate in all directions, very 

 much like those of an engine- turned watch. To these may be added the 

 Suriretta gemma, which presents appearances of a very deceptive character. 

 These appearances, as represented by M. Hartnack, are shown in Fig. 

 118, A, B; the upper part of the valve A being illuminated by oblique 

 light in the direction of its axis, and the lower part by oblique light in a 



1 ' On the Measurement of the Striae of Diatoms,' in " Quart. Journ. of Microsc. 

 Science," Vol. viii. (1860), p. 48. 



2 " Monthly Microsc. Journ.," Vol. v. (1871), p. 162. 



3 See his interesting Memoirs in Vols. ii. and vii. of the " Smithsonian Contri- 

 butions to Knowledge." On Hyalodiscus subtilis, see Hendry, in " Quart. Journ. 

 of Microsc. Science," Vol. i., N.S. (1861), p. 179. 



