JOURNAL OF MICROSCOPICAL SCIENCE. 



DESCRIPTION OF PLATE XIII., 



Illustrating Dr. John Charles Hall's paper on an Easy Method of 

 viewing certain of the Diatomacese. 



Fig. 

 1. — Pleurosigma angulatum ; colour pale chesnut : stri« 52 in '001"; 

 length '0066" to 'OlOO"; this figure is intended to give a. general 

 view of the appearance it presents when ^'iewed with this peculiar 

 illumination, and magnified about 800 diameters. 



2.* — The same magnified about 2,000 diameters. 



3. — Pleurosigma Hipjyocamjms ; colour pale brown : long striaj 32 in 'OOl"; 

 transverse stria? 40 in "OOl" ; length '0050 to '0066"; magnified 

 800 diameters. 



4. — The same magnified 1,200 diameters, showing both the transverse and 

 longitudinal stria? : a portion is also seen in dots. 



5. — Pleurosigma Formosum ; colour light chesnut-brown ; strise 36 in 

 •001"; length -0141 to -0178". 



* Since the present paper was in type, I have seen the very interesting book of 

 Dr. Carpenter " On the Microscope:" in it, at page 307, will be found an engraving, 

 from a photograph of IMr. Wenham, of the Pleurosigma angulatum, as seen under a 

 power of 15,000 diameters. On comparing this with fig. 2, in the present plate, the 

 correctness of Mr. Fleming's delineation will at once be apparent. Dr. Carpenter, 

 after having examined the valve with an objective having an angular aperture of 130°, 

 and very oblique rays, states that its hexagonal areolation becomes very distinct ;" he 

 states also, ' ' that when the object is accurately in focus, the hexagonal area; are seen 

 to be light, and the intervening spaces dark," the reverse being the case when out of 

 focus ; this, of course, does not in any way affect the question, as to the nature of 

 these markings. I consider fig. 2, in the present plate, to be in focus. — J. C. H. 



