2 PROCEEDINGS OP THE 



diverging as though coming from the negative focus N, which is 

 the point to be focussed by the object-glass to make the centre 

 bright. But to do this, the object-glass must approximate the 

 corpuscle, hence it becomes within the focus for the entire cor- 

 puscle. But when this is the case the periphery of the corpuscle 

 is dark, because acting as a double convex lens it causes the 

 parallel rays r' r' r' coming from the mirror to converge and to 

 come to a focus at the point P, above the corpuscle. Now, to 

 make the periphery appear bright, the point P must be focussed. 

 But to focus this, the object-glass must be removed from the 

 corpuscle, since the rays must again diverge before they can 

 again be made to form an image, and in so doing the object- 

 glass is placed "beyond the focus." When this is the case, 

 however, the centre is no longer in focus, and, therefore, ap- 

 pears dark while the periphery is bright. In the opposite posi- 

 tion, or when the objective is within the focus, the centre is bright 

 and the periphery dark. The diagram can easil3^ be carried in 

 the mind's eye, and at once the facts can be thought out with- 

 out troubling with their recollection the memory, which is here 

 particularly apt to be treacherous. Indeed, the speaker said 

 that he could never recollect the circumstances under which the 

 centre was bright and the periphery dark, and vice versa, until 

 he had called to his aid this diagram. And that the exact circum- 

 stances are liable, at least, to escape attention, is seen in the fact 

 that in a work of no less acknowledged value than the seventh 

 edition of "Carpenter's Human Physiology" (1869) is contained 

 a misstatement of the facts. We find, on page 200, the object- 

 glass described as being rather beyond the focus of the micro- 

 scope when the peripher}^ is dark and the centre bright, and 

 within the focus in the opposite appearance — that is, when the 

 centre is dark and the periphery bright. They should be re- 

 versed. In the last edition (1868) of Carpenter on the Micro- 

 scope, however (pages 166, 167), we find the principle applied 

 and the fact correctly stated, though a few lines further we 

 fiud it stated that the hexagonal areola in diatoms appear dark 

 when the surface is slightly beyond the focus, when they are 

 described as hexagonal elevations ; if the latter be the case, then 

 they should ajopear dark ivhen within the focus, as is the case 

 with the periphery of the corpuscle. So, too, on page 110 of the 

 latter volume there is reproduced the same drawing referred to 



