66 On the Similarity between the Bed Blood-corpuscles of Man 
determined by the microscope with certainty whether a given stain 
is composed of human blood or not ; and this fear has been justified 
by some of the notices of his essay which have since appeared in the 
medical journals. 
Now, this subject is one which, from time to time, becomes of 
great importance in criminal cases, and justice, no less than scientific 
accuracy, demands that the microscopist, when employed as an 
expert, shall not pretend to a certainty which he does not possess. 
I suppose no experienced microscopist, who has thoroughly investi- 
gated this subject, will be misled by Dr. Richardson’s paper, but 
there are many physicians who possess microscopes, and work with 
them more or less, to whom a partial statement of facts on such a 
subject as this is peculiarly dangerous ; and the object of the present 
paper is to point out to this class of readers that Dr. Richardson’s 
statement of the case, even if all he claims be granted as true, is, 
after all, not the whole truth : that there are certain mammals — 
among them the dog, the constant companion of man — whose red 
blood-corpuscles are so nearly identical in size with those of human 
blood, that they cannot be distinguished with any power of the 
microscope, even in fresh blood, much less in dried stains ; and 
that, consequently, it is never in the power of the microscopist to 
affirm truthfully, on the strength of microscopical investigation, 
that a given stain is positively composed of human blood and could 
not have been derived from the blood of any animal but man. 
I must do Dr. Richardson the justice to state, at the outset, 
that these facts are well known to him, although, from motives of 
prudence, he has thought proper to be silent with regard to them. 
In a note dated October 19 th, 1874, in reply to one in which I in- 
formed him of my intention to write the present paper, he says, 
“ I should be very much obliged to you if you would add to your 
remarks (in a foot-note or otherwise) that, on communicating with 
me, you found me fully aware of the difficulty of making anything 
more than a differential diagnosis even in the cases I specified, and 
of the impossibility of distinguishing the blood of man from that of 
a monkey or dog, but that I had refrained from giving prominence 
to these facts,” lest an improper use should be made of them in the 
defence of criminals. 
I must, however, entirely dissent from this view of the matter. 
I cannot forget that on more than one occasion in the past, wit- 
nesses summoned as scientific experts have been so misguided as to 
go into courts of justice and swear positively, on the strength of 
microscopical examinations, that particular stains were human 
blood,* and I think the danger that others may do so in the future, 
* Passing by certain American cases, I may refer, in illustration of this state- 
ment, to the celebrated English case, Reg. v. Thomas Nation (Taunton Spring 
Assizes, 1857, p. 279), with regard to which the editor of a London medical journal 
