350 Carpenter on the Microscope. 



The chapter on ' ' Errors of Interpretation" is very important 

 taken in connection with analagous remarks on the " Nature of 

 Surface Marking of Diatoniaceas." Dr. Carpenter observes, 

 " The most common error is that which is produced by the re- 

 versal of lights and shadows resulting from the refractive power 

 of the object itself; thus the bi-concavity of the blood disks of 

 human (and other mammalia) blood, occasions their centres 

 to appear dark when in the focus of the microscope, through 

 the dispersion of light which it occasions ; but when they are 

 brought a little within the focus by a slight approximation of 

 the object-glass, the centres appear brighter than the principal 

 part of the disks. The same reversal presents itself in the case 

 of the markings of the Diatomaceas ; for these, when the surface 

 is exactly in focus, are seen as light hexagonal spaces, separated 

 by dark partitions, and yet when the surface is slightly beyond 

 the focus, the hexagonal area are dark, and the intervening 

 partitions light." While coinciding with the general reason- 

 ing, we would ask whether this passage does not require modi- 

 fication ; cannot a hexagonal appearance be produced by illu- 

 mination and focussing in objects in which it does not really 

 exist ? 



Concerning these much disputed surface markings, Dr. 

 Carpenter now says, that " There can now be no question as to 

 the nature of the comparatively coarse areolation seen in the 

 larger forms, such as Isthmia, Triceratium, and Biddulphia ; in 

 all of which the structure of the valve can be distinctly seen 

 with a low magnifying power and ordinary light. In each of 

 these instances we see a number of areolae, rounded, oval, or 

 hexagonal, with intervening spaces symmetrically disposed. . . . 

 That the areolae are really depressions is suggested by the ap- 

 pearances presented by the surface when the light is obliquely 

 directed, and it may also be inferred from their aspect when 

 viewed by the black ground illumination, since the areolae are 

 then less bright than the intervening spaces." After adducing 

 other reasons, for the " depression" interpretation, the author 

 passes to the consideration of the more delicate workings on 

 minuter diatoms, and especially on those of the genus Pleuro- 

 sigma, and he tells us when a P. angulation is examined with 

 an objective of one-twelfth of an inch focus, and an angular 

 aperture of 170 D , and a magnifying power of 1200 diameters, 

 it presents a hexagonal areolation* somewhat resembling that 

 of Tricerstinm. We suppose other observers will still deny the 

 accuracy of this hexagonal appearance, but the majority will 

 probably coincide with Mr. Wenham/s present opinion, and 

 that now avowed by Dr. Carpenter, that the Pleurosigma areolae 

 1 ' are minute tubercular elevations.'" 



* There is some mistake here in the reference to Plate II. 



