126 Dr. G. C. Wallich on the Markings of the Diatomacea 



case, the more decidedly are they found to correspond with what 

 has been already seen. Thus, if we examine Pleurosigma angu- 

 latum (one of the easier tests) with an objective of 5 inch 

 focus and 75° aperture, we shall see a double series of interrupted 

 lines, crossing each other at an angle of 60°, so as to have be- 

 tween them imperfectly defined lozenge-shaped spaces. When, 

 however, the valve is examined with an objective of ~ inch 

 focus, having an angular aperture of 130°, and is illuminated 

 by oblique rays, the hexagonal areolation becomes very distinct ; 

 and if a photographic representation obtained by such a power 

 be itself enlarged by photography, as has been accomplished by 

 Mr. Wenham, the appearance represented (by the diagram, loc. 

 cit.) is obtained, which is in all respects compai-able with that 

 presented under a low power by the valve of Triceratium or 

 IsthmiaJ" 



In the ' Micrographic Dictionary' we find observations nearly 

 to the same eflfect. But a remarkable paragraph occurs at page 

 221 (new ed.), in the article " Diatomacese." After denoting the 

 varying phases a Diatomaceous valve may be made to assume 

 under different powers and modes of illumination, it thus con- 

 cludes: — "If the condenser and stop be not exactly centrical, 

 or the surface of the valve be not flat, the true form of the dots 

 will be replaced by some other : thus hexagonal dots may be 

 made to appear triangular, quadrangular, &c., and those dots 

 which cannot be conceived to be really hexagonal may be made 

 to appear so," — Pleurosigma balticum being here referred to 

 amongst the figures. 



I duly appreciate the difficulty of having to deal with state- 

 ments supported by such unquestionably high authority, — more 

 particularly when they seem to carry with them the proof de- 

 rived from the photographic art. But the latter testimony ap- 

 pears to me by far the most assailable, and I have adduced the 

 paragraph just quoted in support of my assertion. For if, by 

 modification in the focussing of a valve, hexagonal dots may be 

 made to appear triangular or quadrangular, and those dots which 

 cannot be conceived to be really hexagonal may be made to 

 appear so, it follows that by similar modification under the pho- 

 tographic appendage to the microscope, these figures may be 

 respectively reproduced according to the fancy of the operator. 



Although the evidence derived from a photographic picture 

 may safely be relied on under ordinary circumstances, it at once 

 loses its value in the case before us, the image thrown on the 

 prepared plate or paper being of necessity identical with that 

 impressed upon the eye of the observer when adjusting the 

 object to be depicted, and whatever he sees being, as a matter 

 of course, reproduced in the photographic picture. 



