Scientific Lectures. 31 



moving points in tlie water, when placed under the microscope, to 

 be loaded down bj stipitate diatoms, unicellnlar vegetables having 

 stems, or by tnbicolar animalcules, little animals which construct for 

 themselves tubular dwellings, without nevertheless seeming to occa- 

 sion the animal which carries them the slightest inconvenience. 

 ' The microscope furnislies the means of discovering the chemical 

 nature of substances when in quantity too minute to be treated by 

 ordinary methods of analysis. It does this by detecting character- 

 istic forms of crystallization, as in investigations for the detection of 

 poisons, organic or mineral, or by showing the action of reagents 

 precisely as when testing on the large scale in the usual way, or by 

 showing the peculiar optical effects produced by polarized light, or 

 by simply making manifest the mechanical structure of the object 

 under investigation. In questions of medical jurisprudence the 

 information it furnishes, is thus often decisive. Its testimony as to 

 the nature of stains supposed to be blood, and as to the question 

 whether the blood, if present, is that of man or beast, is beyond 

 appeal. Mr. Gosse mentions a case wliich strikingly illustrates this 

 statement. A man suspected of a murder was foimd with a bloody 

 knife in his possession. He accounted for the stain by saying that 

 he had been using the knife for cutting beef. The implement was 

 given to an expert with the microscope, who pronounced that the 

 blood was human blood, and tliat it had proceeded from a living body 

 and not fi'om dead flesh. He discovered also, mingled with the 

 blood, certain vegetable fibers, which he pronounced to be cotton, 

 and which were found to agree witli the material of the murdered 

 man's neckcloth. And he found also present numerous tesselated 

 epithelial cells — such cells as line the mucous membrane of the 

 throat, and of no other part of the body. It was evident that the 

 knife had been used to cut the throat of a human being, who wore a 

 cotton neckcloth at the time. The decision of the instrument as to 

 the presence or absence of poisons, is often equally conclusive. In 

 one instance, in England, a criminal was convicted on the evidence 

 of the sand which he had brought on his boots from the scene of his 

 ci-ime, and which the microscope unerringly traced to its true locality. 

 In the detection of the adulteration of drugs, groceries and other 

 articles of daily commerce, the microscope is infallible. At the oflice 

 of the Surgeon-General, in Washington city, all the supplies pur- 

 chased or tendered for purchase, for the military hospitals and medi- 

 cal stores of the army, are subjected to constant and severe micro- 



