42 PROGRESS OF MICROSCOPICAL SCIENCE. 



The membrane itsolf is about this length, and ^V*^ of an inch wide, 

 tapering towards the proximal end. The smaller specimen is ^jV^^^ ^^ 

 an inch in entii'e length, and -gjyih. wide. If these young forms had 

 not been in connection with a mature graptolite, they would have 

 been considered to belong to the genus Diplograjjsus, but it would 

 have been impossible to refer them to any species. In their present 

 position he thinks we may without hesitation infer that they are the 

 young of the grajitolite with which they are associated. That they 

 have not yet entered upon independent existence we cannot conclude ; 

 for they are in different stages of growth, and young graptolites are 

 frequently met with in a less advanced state than either ; indeed, on 

 the same piece of shale there are several young graptolites referable 

 to the same species, and no more developed, some even less so. — 

 Annals and Magazine of Nat. Histonj, May. 



When is a Blood Corjmscle in Focus? — Dr. Tyson has* a very in- 

 teresting note, accompanied by a diagram, which we regret we cannot 

 reproduce, on this optical and physiological subject. After explaining 

 the diagram, he says it can easily be carried in the mind's eye, and at 

 once the facts can be thought out without burdening with their recol- 

 lection the memory, which is here peculiarly apt to be treacherous. 

 Indeed, he said he could never himself jiromptly recall the circum- 

 stances under which the centre had been bright and the periphery 

 dark, and vice versa, until he had called to his aid this diagram. And 

 that the exact truth is liable at least to escape attention, is seen in 

 the circumstance that " in a volume no less highly valued than the 

 seventh edition of Carpenter's ' Human Physiology,' 1869, is con- 

 tained a misstatement of the facts. We find here, on page 200, 

 the statement that the corpuscle is rather beyond the focus of the 

 microscope when tho perij^hery is darh and the centre bright, and ivitMn 

 the focus in the opposite api^earance — that is, when the centre is darh 

 and the perixjhery bright. The reverse is correct. In the last edition 

 of Carpenter (1868) ' On the Microscope,' however (pages 166, 167), 

 we find the principle applied, and the fact correctly stated, though a 

 few lines farther we find it asserted that the hexagonal areolse in dia- 

 toms appear darh when the surface is slightly beyond the focus, though 

 they are described as hexagonal elevations. If this latter be the case, 

 then they shoidd appear darh lohen within the focus, as is the case with 

 the periphery of the corpuscle. So, too, on page 710 of this latter 

 volume there is reproduced the same drawing referred to in the text- 

 book on physiology, but with the description reversed, and therefore 

 correct. The corpuscle is, however, described as in focus when the 

 perij)hery is in focus, whereas we have presumed that the entire cor- 

 puscle is in focus when there is least shadow. Of the other text-books 

 now within our reach, Dalton has it correctly on page 214 of his third 

 edition ; Flint, Kirke, Eauke in his ' Grundziige der Physiologic,' and 

 Eollett in Strieker's ' Handbuch der Lehre von den Geweben,' refer 

 to the reversal of light and shadow, but do not state the circumstances 

 under which it takes place ; Marshall makes no allusion to it." 



* Philadelphia ' Medical Times.' 



