Photograi)htj to Micrometry. By Br. Woodward. 145 
its cover ; but if absolute values are aimed at, the micrometer must 
be compared v^ith a recognized standard, and its constant error 
ascertained. The stage-micrometer used in the present series of 
experiments is the same described in a former paper * It is ruled 
in hundredths, thousandths, and five-thousandths of an inch ; and, 
so far as I have been able to ascertain by repeated comparisons of 
its parts with each other, the several divisions of each kind are 
equal throughout, five of the five-thousandths being equal to any 
one of the thousandths, and ten of the latter to any one of the 
hundredths. 
On comparison with a standard scale belonging to the United 
States Coast Survey, by the contact method of Welcker, this micro- 
meter proved, as I stated in the paper just cited, to have a constant 
error of -|- 1 * 945 per cent. ; that is, its lines are very nearly 
2 per cent, too far apart ; and this correction must of course be 
applied in the measurement of the photographs, as it was in the 
measurement of the corpuscles as seen in the microscope, which 
were published in the paper referred to. 
The micrometer having been selected, it may be used for the 
measurement of blood dried in a thin film, of fresh blood in the 
moist state, or of dried stains soaked out by any selected method. 
In the first case the fresh blood should be spread on the micro- 
meter by means of the edge of a glass slide, as proposed by 
Dr. Christopher Johnston, of Baltimore. A camel's- hair pencil 
is sometimes used for the same purpose ; but it is a clumsy device, 
which no one who has been initiated into the proper method will 
ever employ. The blood thus spread may be photographed as seen 
uncovered, with a dry objective ; but the best results are obtained 
with immersion objectives, to use which the specimen must of 
course be covered with a suitable piece of thin glass ('OOS" to 
•008" thick). 
After two or three negatives have been made from the sample 
first selected, it is washed off, and a fresh specimen substituted, and 
so on. If it is desired to photograph fresh blood, a drop is put on 
the micrometer, a thin cover (of the thickness named) dropped on 
and allowed to press out the blood to the thinnest possible film. 
Excellent results may thus be obtained with the micrometer, 
almost as sharply defined as in the case of dried blood. If dried 
stains are to be examined, as in criminal cases, the fragments 
of dried blood are to be soaked out, by any of the approved 
methods, on the same micrometer, just as if on an ordinary glass 
slide ; and when a satisfactory preparation has been made, it may 
be photographed, together with the micrometer, as in the former 
* J. J. Woodward, " On the Similarity between the Red Blood-corpuscles of 
Man and those of certain other Mammals, especially the Dog," &c., ' American 
Journal of the Medical Sciences,' January 1875, p. 151, and the ' Monthly Micro= 
scopical Journal,' February 1875, p. 65. 
