68 On the Similarity between the Bed Bloocl-corjpuscles of Man 
have examined, any of the numerical data from which this average 
size was deduced. 
In the table of measurements appended to Gerber's ' Elements/ 
in which, for the first time, he gave " mean or average sizes " (in 
previous papers he had only recorded " common sizes," occasionally 
supplementing these by the extremes observed), Mr. Gulliver ex- 
plained his method of arriving at the average size, as follows : " The 
common-sized corpuscles are first set down, then those of small and 
large size, and lastly the average deduced from a computation of the 
whole." * In this table the measurements for the common dog are 
given as follows : t 
(1-4000 of an inch. 
Common sizes . . . . \ 1-3500 „ 
(1-3200 „ 
Small size 1-4570' „ 
Large size 1-2900 „ 
Average 1-3542 „ 
Where the "average" is simply the arithmetical mean of the 
several fractions given above, it can hardly,^I think, be accepted as 
the true average size, since as much weight is given, in this mode 
of calculating, to the rarer as to the more frequent forms. Ac- 
cordingly, it is not surprising that we find in a former paper J 
measurements which do not accord very closely with this average. 
*' Domestic dog, old mongrel. Common diameter of corpuscles, 
l-4000fch to 1-32 00th of an inch. Foxhound puppy, one day old, a 
bitch, l-3000th and l-2666th, the most common diameter of the 
corpuscles. Foxhound puppy, twelve days old, a bitch. Most 
common diameter of the corpuscles l-3000th and l-2885th of an 
inch. Extreme sizes l-4000th and l-2666th. Mongrel puppy, 
four months old, a bitch ; all the following diameters common, viz. 
l-3693rd, l-3554th, l-3429th, and l-3200th." The measurements 
for the second and third of these animals are about as much larger 
than those for the human species as the others are smaller. 
It is interesting to know just how Mr. Grulliver's measurements 
were made. He tells us he used a glass eye-piece micrometer so 
adjusted that the divisions had a value of ^xrV o^h of an inch each. " If 
one space and a quarter of this micrometer were occupied by a 
single globule, this would of course measure ^?V^th of an inch ; if 
three equally-sized particles lying in a line, and touching at their 
edges, covered three spaces and a half, the diameter of each of these 
would be ^Vijth, if four spaces Wo-xjth of an inch."§ The objectives 
used were an eighth by Boss and a tenth by Powell. || It is not 
* Appendix to Gerber's 'Elements,' cited above, p. 1. 
t hoc. cit, p. 38. 
X ' London and Edinburgh Philosophical Magazine,' vol. xvi. (1840), p. 28, 
>^ ' London and Edinburgh Philosophical Magazine,' vol. xvi., p. 24. 
II Loc. cit., pp. 24 and 105. 
