206 Original Articles. [April, 



reference to the detection of minute traces of blood in criminal inves- 

 tigations, and therefore have directed a great amount of attention to 

 that point alone. Such inquiries almost necessarily fall into the 

 hands of a microscopist ; and, as I shall show, the apparatus I have 

 described will enable anyone to detect with certainty most marvel- 

 ously minute traces of blood, when the usual methods would entirely 

 fail. That merely chemical tests are generally suspicious, and often 

 not to be trusted at all, is freely admitted by my friend Dr. Allan, 

 who has had much experience in such inquiries ; and, of course, if the 

 red globules have been destroyed, the microscope alone is of little or 

 no use. In such cases the method I now propose comes to our aid, 

 and enables us to detect as small a quantity as nxooth of a grain of the 

 colouring matter of blood, left on a fabric from which apparently all 

 traces have been washed, or which only shows a slight discoloration. 

 If all has been washed out its detection is no longer possible ; for it 

 is this colouring-matter itself, and not any action on the material, that 

 proves the former presence of blood. 



Since in studying the spectra of blood, it is important to be able 

 to operate on small quantities, I have employed cells, cut from a 

 barometer tube, having an external diameter of about ^ inch, an inter- 

 nal diameter of |, and h inch long, slightly polished at both ends. 

 These may be fixed with Canada balsam in the centre of pieces of 

 glass about 1'6 square, or what is still better, in the centre of the 

 longer side of triangular pieces, formed by cutting such squares along 

 a diagonal, so that two may be placed side by side in front of the slit, 

 and the sj>ectra of two solutions compared together at one view. 

 Liquids can be readily introduced into such cells by means of a mo- 

 derately stout platinum wire ; and, when rather more than full, so 

 that a little spreads over the top of the cell, a piece of thin glass can 

 be easily put on without inclosing any bubble, and the surplus re- 

 moved with blotting-paper. Capillary attraction keeps the glass on, 

 and evaporation takes place so slowly, that everything remains for a 

 considerable time in a proper condition; and, if desirable, the glass 

 may be fixed on by means of gold-size, and the solution kept for 

 weeks. Placing such a cell in front of the slit, as at a in Fig. 1, the 

 light passes through the thin glass at the top, through the column of 

 liquid and the thick glass at the bottom, and then through the narrow 

 slit to the prism. Cells of less diameter than \ of an inch might 

 be used, but it is difficult to fill and empty them. Even when \ in 

 diameter, if it be requisite to transfer the solution to a watch-glass, it 

 is well to have a piece of such tube as is used for spirit-thermometers, 

 drawn out at one end and bent at a right angle, so that it may be 

 introduced to the top of the cell, and the liquid removed by blowing 

 down the tube. 



The exact position of the dark absorption-bands in the spectra 

 obtained from blood being a very important character, it is necessary 

 to make use of the micrometer ; and I have found that for this pur- 

 pose it is well to arrange the instrument so that the width of the 

 entire spectrum is about 12, as shown by the divisions on the left 

 hand side of Fig. 2. I shall adopt this method of measurement in all 



