1865.] Sorby on Detection of Blood-Stains by Spectrum-Analysis. 215 



be accomplished by other means, or a conclusion formed from other 

 considerations. 



If the quantity dissolved out at first gave a very faint spectrum, it 

 would be well not to attempt to see the spectra Nos. 5, 6, and 7, but to 

 act on the solution with citric acid, filtering, if requisite, and then to 

 add ammonia and protosulphate of iron, so as to obtain No. 8. The 

 material itself might also have so acted on the blood that no other 

 spectrum could be seen. It is, however, so characteristic, that, even 

 then, one could scarcely hesitate in deciding that the stain was due 

 to blood. 



If the material had evidently been washed, so that no spectrum 

 could be obtained from a simple solution, having first ascertained 

 what effect citric acid has on the dye, I should digest a stained por- 

 tion in a solution of ^rd or Jth of a grain of citric acid in a few drops 

 of water, so as not to have more liquid than will fill the cell, filter, if 

 requisite, add excess of ammonia, and then protosulphate of iron. 

 Unless very thoroughly washed out whilst the blood was quite fresh, 

 no mere washing with hot or cold water without brushing or other 

 mechanical means being adopted to dislodge the minute insoluble 

 particles, will so completely remove the stain that it could not be 

 detected in this manner ; except, indeed, any of those foreign sub- 

 stances be present which precipitate the colouring-matter in a form 

 insoluble in citric acid and ammonia. 



Supposing a clear coloured solution be obtained, which gives a 

 different spectrum or set of spectra to those characteristic of blood, it 

 might be desirable to ascertain the nature of the substance to which 

 they were really due. I have already described the spectra of a few 

 colouring-matters, but in all cases I strongly recommend the study of 

 the spectra of any substances likely to occur in any particular in- 

 stance. Even if blood has been detected, this would also be desir- 

 able, since an opinion coidd then be given with more confidence. 



My conclusion is, that, even in cases where the usual tests would 

 fail, the detection of minute traces of blood need not in general be a 

 weak point in the evidence of crime. Having proved it to be present, 

 the question of its having been human or not must be decided by 

 other considerations ; and thus as a positive proof the evidence may 

 be equivocal : but if a stain supposed to be due to blood, which, if 

 it were, would certainly give the characteristic spectra, fail to do so, 

 the negative evidence would be decisive. 



7. Conclusion. 



It is, of course, at first, always difficult to know to what subjects a 

 new form of apparatus may be usefully applied ; but I trust that the facts 

 I have described will suffice to prove that, for so simple an addition, 

 the resources of the microscope as an instrument of research are 

 greatly increased by our being able to use it also as a spectroscope. 



