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MICROSCOPY. . 251 
migration of the white corpuscles of the blood through the tissues 
of the body has been demonstrated ; the diameter of the body of 
the young filaria is considerably below that of the corpuscle ; hence 
with the brisk, wriggling movements of life, the possibility of their 
passage through a mucous membrane, especially through the soft 
granulations of an ulcer, is quite within the bounds of reality. 
Based upon the facts we know, we may in imagination follow them 
from a mucous tract (e. g. the intestine) to a lacteal or blood ves- 
sel; they follow the eourse of the circulation, growing on the 
pabulum of the blood of the host, and easily passing with the 
corpuscles through the capillaries; soon their size unfits them to 
traverse every viscus, and the minute capillaries of the lungs act 
as a sieve to retain them in the venous circulation; they copulate 
and the females become fecund ; a young brood arises to continue 
the race, provided accidental causes, such as mechanical block- 
ing up of important, blood-vessels by the parent worm, do not de- 
termine the death of the host. By this hypothesis the ingress of 
h 
individuals capable of arriving at maturity is explained, while the 
countless hordes of young are rendered lucid only by the pres- 
ence of one or more parent worms within the vascular walls. 
These parent worms after producing their progeny may possibly 
die and disintegrate, and. so account for their absence, or non- 
discovery, in hosts teeming with the young brood.” The presence 
of the, parent worm is attributed suggestively to the ingestion of 
water or under-cooked flesh containing them. 
Finpinc tHe CuemicaL Focus IN PHOTOMICROGRAPHY.—Prof. 
H. A. Rowland has suggested, at the Troy Scientific Association, 
the simple expedient of laying a broad flat object, as for instance 
& microphotograph or a large transparent section, obliquely upon 
the stage, so that one edge shall be considerably higher than the - 
other.. The objective is then carefully focussed for some one well- 
marked portion of the object, and a photograph taken which will 
_ Of course show the best definition at some other portion. The in- 
strument is next focussed for the point in the object which in the 
photograph is best defined, and the distance apart of these two 
planes, measured by the fine adjustment wheel, being the distance 
of the chemical from the visual focus, is a correction which may 
always be employed in photographing with the same objective, the 
_ lens being focussed as usual by sight and then turned out of focus 
Rs ; os 
