ACCIDENTS BY RAIL AND SEA. 181 



when a multitude of persons are to be successively examined. A simple 

 calculation shows us in fact ttat an examination requiring one day by 

 our method would require two months by Seebeck's. 



It is but just to. acknowledge that it was only by weighing the results 

 obtained by Seebeck's method and following the Young-Helmholtz 

 theory, as well as the principles we have indicated as indispensable to 

 a practical method, that we have succeeded in formulating our own 

 method, such as we shall explain it in what follows. We also will re- 

 mark that it is very simple and easily mastered ; but we think this is 

 likewise often the case with all that is useful and practical, and that 

 simplicity offers great advantages. We prefer this method because it 

 seems to us more than any other to fulfill the conditions we have 

 pointed out as necessary to a practical method, namely, certainty, rapid- 

 ity, and convenience. The only inconvenience of any moment besides 

 those it has in common with a greater part of the others is that it re- 

 quires daylight. It can undoubtedly be used by artificial light (elec- 

 trical and calcium lights, and certain arrangements of lamp-lights with 

 blue glass), but this causes much loss of time. 



After this rapid sketch of the general principles of the method, we 

 will proceed to give its details, and shall not fail to mention generally 

 the reasons why, amongst several possibilities, we have selected this or 

 that process. 



2. — THE MATERIAL AND ITS ARRANGEMENT. 



Our method demands neither costly apparatus nor a special place for 

 the examination. The only necessary elements are a number of variously 

 colored objects. It consists in taking one from a number of objects pro- 

 miscuously thrown together, and asking the person examined to select 

 from amongst them all the others corresponding with the first in color. 

 With regard to the colored objects, it of course matters little in principle 

 what their nature is, as, in the main, the method never changes, no mat- 

 ter what the kind selected. But, practically, the choice is by no means 

 a matter of indifference. Among the ordinary objects suggested, and 

 also used for the purpose, are pieces of colored paper, glass, or silk, 

 or Berlin wool, etc., the last of which seems to us the best, for the 

 following reasons: One of the chief advantages of Berlin wool is, that 

 it can be procured in all possible colors corresponding to those of the 

 spectrum, and each in all its shades, from the darkest to the lightest. 

 Such selections may be found in trade, and are easily procured when and 

 where desired. It can be used at once, and without any preparation 

 for the examination, just as delivered from the factory. A skein of 

 Berlin wool is equally colored, not only on one or two sides, but on all, 

 and is easily detected in the package, even though there be but one 

 thread of it. Berlin wool is not too strongly glaring, and is, moreover, soft 

 and manageable, and can be handled, packed, and transported as desired, 

 without damage, and is conveniently ready for use wherever needed. 



