78 OBSERVATIONS ON FILARI^. 



directs. Whence, then, all this difficulty, as conjured up by Dr. 

 Magalhaes ? 



Dr. Lewis, in perfect consistency with his theory, that the Fila?'ia 

 sangmnis hominis is enveloped in an extremely delicate tube, closed 

 at both ends, within which it is capable of elongating and shorten- 

 ing itself, speaks of the shortening of the Filaria, and of the visi- 

 bility of the involucrum as synonymous, as convertible expressions 

 for one and the same phenomenon. How, indeed, could it be other- 

 wise ? The problem is a very simple one. Inside, but unattached 

 to, a very diaphanous tube, closed at both ends, there exists a 

 comparatively opaque body, capable of entirely filling it. Under a 

 low microscopic power nothing is seen but the opaque body. Under 

 even a high power, so closely fitting is the enclosing tube, and 

 of such extreme tenuity, that it cannot be distinguished as a sepa- 

 rate existence, so long as the opaque body continues to fill the 

 ■whole of it, coming into view only when that opaque body is 

 withdrawn. 



As the tube is a short one, that withdrawal can be effected only 

 by the shortening of the opaque body itself ; which shortening is, 

 therefore, the exact measure of the part of the tube rendered visible. 

 This shortening, like the opaque body itself, might be seen under a 

 power too low to bring into visible recognition the pellucid tube ; 

 but, under no possible circumstances could the empty tube become 

 visible, and the shortening of the opaque body remain unseen. The 

 difficulty of seeing the involucrum consists solely in its extreme 

 tenuity. The difference in breadth of a blood corpuscle, as it squeezes 

 itself through a space narrower than its transverse diameter, is an 

 appreciable quantity, and is, assuredly, not the ten-thousandth part 

 of the ordinary length of the involucrum as seen in Lewis's demon- 

 strations, and, consequently, not the ten-thousandth part of the 

 shortening undergone by the FiJaria, as Lewis says, from one moment 

 to another. Now, these two correlative quantities, the shortening 

 of the Filaria and the proportional visibility of the involucrum, that, 

 were Lewis's theory the true explanation of the phenomenon, ought 

 to be, and are spoken of by him as being, the exact counterpart of 

 one another, are, in the experience of every observer in Bahia, seen 

 to be altogether independent the one of the other ; or, rather, one 

 of them is seen to be altogether non-existent. We have here, all of 

 us, seen the so-called involucrum of all imaginable lengths, from 

 under one-thousandth to over one half the length of the Filaria, and 



