WELCKER, ON MICROSCOPIC MEASUREMENTS. 247 
It occasionally happens in microscopic investigations, that 
minute morphological elements of unknown chemical nature 
occur within a tissue, but which are too minute or too 
difficult of isolation to be subjected to chemical analysis. May 
it not, in such a case, when the “ true” and the “ apparent’ 
thickness of the corpuscles are known, perhaps be possible 
to arrive at some conclusion as to their chemical constitu- 
tion? Let us, by way of example, take a case in which it is 
doubtful whether an inclosed corpuscle be albuminous or fatty. 
Let the apparent thickness of the corpuscle = 0:0030 mm., 
and the true thickness (ascertained by common measure- 
ment of it in profile) = 0°:0042mm. The object in this case 
cannot be fat,—for in fat the true diameter stands to the 
apparent in the proportion of 0:0045 to 0°0030. But this 
ascertained refractive power would correspond with that of 
albumen (0:0030 : 0:0042 = 100: 140). 
But, although considerations of this kind maylead to the pro- 
spect of a new, and not altogether useless method of diagnosis, 
its application will be found to be much limited by the fol- 
lowing conditions. In the first place, the method would 
become more and more uncertain, and ultimately wholly 
useless, in proportion to the greater minuteness of the ob- 
jects. And, also, when the object has rounded sur- 
faces, it is not, for the reasons already adduced, at all 
adapted to determine the refractive power or the apparent 
thickness of the object. 
From the above exposition, I am by no means of opinion 
the mode of measuring by the vertical movement of the tube 
of the microscope, for either of the purposes proposed, is 
capable of any extensive application. My object has rather 
been to test the applicability of such a mode of measurement 
in the general method, and to establish it in some particular, 
may be rare instances. 
