OF THE DIATOME^. 43 



A few directions on the mode of manipulation may be useful. As the fumes 

 of the hydrofluoric acid, if they reach the lenses, would greatly injure them, 

 it is advisable to protect the front face of the objectives by temporarily con- 

 necting to them a thin plate of mica by Canada balsam, as mica resists the 

 action of hydrofluoric acid much better than glass. I prepare the cell in 

 which the solution is to take place by cementing a plate of mica to a glass 

 slide, and then cover all its surface, except a central small disc, with wax. 

 On this disc, which forms a cell, the shells are put with a dish of water, and 

 after adding a drop or two of acid by means of a dropping-rod of silver or 

 platinum, the cell is covered with another plate of mica, and the slide is then 

 placed imder the microscope." 



Some markings of the surface, apparent only as striae under inferior magni- 

 fying powers, are in several genera resolvable, as before noticed (p. 40), into 

 rows of rounded dots, e. g. in Pleurosigma ; and in consequence such specimens 

 have been employed to try the relative powers of microscopes, and are spoken 

 of as ' test objects.' But the powers of microscopes have been more severely 

 tested of late years, by the endeavour to ascertain whether such dots are eleva- 

 tions or de]3ressions of the surface, and, as might be expected, the dissension 

 on this matter has equalled that respecting the central band and umbilicus. 



Dr. J. W. Griffith is in favour of their being depressions {Proc. Roy. Soc. 

 1855). He argues that, as the markings '^ are evidently depressions in the 

 genera and species with coarsely marked valves (IstJimia, &c.), we should 

 expect from analogy that the same would apply to those with finer markings 

 (those viz. in dispute, Gyrosigma, Pleurosigma, and others). And this view 

 receives further support from the fact that under varied methods of illumina- 

 tion corresponding appearances are presented by the markings when viewed 

 by the microscope — from those which are very large, as in Isthmia, through 

 those of moderate and small size, as in the species of Coscinodiscus, down to 

 those in which they are extremely minute, for instance in Gyrosigma, 

 &c. The angular (triangular or quadrangular) appearance assumed by the 

 markings arises from the light transmitted through the valves being un- 

 equally oblique ; this may be readily shoAvn in the more coarsely marked 

 valves {IstJimia, Coscinodiscus), which present the true structural appearance 

 when the light is reflected by the mirror in its ordinary position, and the 

 spiuious angular appearance when the light is rendered oblique by moving 

 the mirror to one side." Another statement is put forward by the same 

 author in the MicrograpMc Dictionary (Introduction, p. xxxiii) in support 

 of his oj)inion, viz. " that the line of fracture of the broken valves passes 

 through the rows of dots on the dark lines corresponding to them, showing 

 that they are thinner and weaker than the rest of the substance. Had these dots 

 represented elevations, the valves would have been stronger at these points." 



The more prevalent opinion, however, is, that these delicate dots in rows 

 are elevations of the sm-face. Mr. G. Himt {J. M. S. 1855, pp. 174-175) 

 adduces an observation to demonstrate this fact. He found that on a speci- 

 men of Pleurosigma being moistened, the markings were almost entirely 

 •obscured, but that on the application of a gentle heat " the moisture slowly 

 retreated, lea\ing patches of the shell diy, and with the markings as dis- 

 tinct as before." On observiaig these dry parts of the sheU, they were seen to 

 be uniformly bounded by straight ]hiQB, parallel to the two directions of least 

 distance of the dots. " Now (continues Mr. Hunt), on the supposition of 

 these Httle dots being elevations, the phenomenon appears to me easily ex- 

 plicable on the princijjle of capillaiy attraction. We can readily conceive 

 the moisture clinging from one dot to another ; and it would always have a 

 tendency to arrange itself in lines parallel to the directions of least distance. 



