DIATOMACE.E. 



[ 203 ] 



DIATOMACE^E. 



although these probably consist of rows of 

 dots ; these very difficult valves require the 

 use of an Amici's prism (INTRODUCTION, 

 p. xviii). 



We have already stated (INTRODUCTION, 

 j). xxiii, /.) that the dots consist of depres- 

 sions. In reviewing the considerations 

 establishing this point, we may divide the 

 valves into those which exhibit the dots by 

 ordinary light, and those which require ob- 

 lique light and the use of stops. 



In those^isible with ordinary light (PI. 13. 

 fig. 29; PI. 18. fig. 32, &c.), the valves are 

 thinner and weaker at the parts occupied by 

 the dots, so that the line of fracture corre- 

 sponds to these parts; and the depressions 

 are distinctly visible at the edges of the 

 curved portions of the valves (PI. 13. fig. 2 b). 

 In those requiring the use of oblique light 

 and stops, the line of fracture also corresponds 

 to the rows of dots, provided the light be 

 equally oblique on all sides; and the same 

 appearances are presented by the dots in 

 both cases, beginning with those in which 

 they are very large (as in Isthmia], to those 

 of moderate and small size (as in the species 

 of Coscinodiscus], down to those in which 

 they are extremely minute (as in Gyrosigma, 

 &c.). Moreover, analogy aifords a strong 

 confirmatory ground, for the Diatomaceae 

 form a very natural family ; and if the dots 

 are depressions in some genera, we might 

 expect them to be so in the others. 



The explanation of the manner in which 

 oblique light renders the dots visible, has 

 been given under ANGULAR APERTURE. 

 Some objections have been made to a part of 

 that explanation, but as they are founded on 

 a want of acquaintance with the subject, we 

 have not considered it requisite to notice them. 



The method of determining the structure 

 of the frustules of the Diatomacea3 is the 

 same as that of microscopic bodies in general, 

 and has been laid down in the INTRODUC- 

 TION, p. xxxii. The presence or absence of 

 a gelatinous envelope or a stipes should first 

 be determined. The general form of the 

 frustules, both in the front and side view, is 

 next examined, which should be done while 

 they are immersed in water; the frustules 

 being made to roll over by gently moving 

 the glass cover with the point of the mounted 

 needle, the eye being kept upon the object, 

 and a somewhat low power used. The frus- 

 tules should then be prepared, and examined 

 when dry as to their markings. Perhaps 

 these may be visible by ordinary light; if 

 not, the mirror should be turned on one side 



as much as possible to obtain the effects of 

 oblique light. If lines then become visible, 

 it does not follow that the valves are marked 

 with lined structures such as grooves or 

 ridges ; because the shadows of rows of dots 

 may become extended into lines under 

 oblique illumination, in any direction in 

 which the dots will form a linear series. This 

 point must, however, be decided by exami- 

 nation with the aid of the condenser, stops, 

 &c. ; and if the valve be much curved, it 

 must be crushed, so as to obtain a fragment 

 as flat as possible. The markings upon the 

 most difficult valves can only be brought 

 out by using extremely oblique light, reflected 

 either from the mirror brought as close be- 

 neath and as much on one side of the stage as 

 possible, or from the Amici's prism. The field 

 will then appear black or nearly so, the valve 

 having frequently a bluish appearance ; this 

 extreme obliquity of the rays of light being 

 essential, to allow of one set being thrown 

 out of the field (see ANGULAR APERTURE). 



In using very oblique unilateral light, 

 spurious rows of parallel lines are often seen, 

 not only upon the valves of the Diatomaceas, 

 but upon objects not possessing a lined 

 structure, as many crystals, &c. These can 

 only be distinguished from those connected 

 with the presence of dots, by their not being 

 resolvable into dots, their greater coarseness 

 and their variability in number (in a given 

 space) under different kinds of illumination. 



If the direction of the lines changes with 

 the variation of the position of the valve to 

 that of the incident light, it may be pretty 

 surely predicted that the lines are spurious, 

 and that the condenser and stops will effect 

 their resolution into dots. 



The prepared valves of the Diatomaceae 

 frequently appear coloured when dry, the 

 colour vanishing when they are moistened. 

 This colour arises from iridescence, and not 

 from the presence of pigment or other 

 colouring matter (INTRODUCTION, p.xxx,3). 



Collection. In collecting the Diatomacea3, 

 a number of phials (1 to 2 oz.), with wide 

 mouths and furnished with corks, must be 

 provided, in which they may be brought 

 home. The mouth of the bottle being closed 

 with the thumb and brought as closely as 

 possible to the masses of them in the water, 

 on removing the thumb, the water will enter 

 and carry the Diatomacese with it into the 

 bottle. A spoon is frequently of use in 

 removing layers of them from the bottom of 

 the water, or from pieces of woodwork, &c. 

 immersed in the water. Many of them are 



