152 Mr. Alex. Hodgkinson on 



sequence of the play of colours, or the relation of these 

 colours to the direction of the iridescent light, i.e., whether 

 produced by perpendicular or oblique illumination. Here is 

 a description of the tufted neck humming bird, TrocJiilns 

 omatits, taken haphazard from a well-known work : — 

 " The throat is of a fine green colour, variable in different 

 lights to a golden hue with a yellow or brown metallic 

 lustre, and below that the whole of the belly is a rich 

 brown, glossed with green, and golden." Such descrip- 

 tions as the above, which happen to be the first I met 

 with in seeking for an instance, are vague, and fail to 

 give a definite idea of the appearance of the object. But 

 vagueness in the description of these objects is not the only 

 result of the changing character of their colours. As might 

 be expected, where such variation in appearance exists, the 

 descriptions of different authors are almost as variable as the 

 colours. Few attempt descriptions without acknowledging 

 the hopelessness of the task. Thus Jardine, after des- 

 cribing this humming bird, CJiryslampis mosquilus, remarks: 

 " It is impossible to convey by words the idea of these tints, 

 and having mentioned those substances to which they 

 approach nearest, imagination must be left to conceive the 

 rest." And I adduce this quotation as fairly expressing the 

 feeling of naturalists in reference to the description of 

 iridescent objects generally. Recognizing the admitted 

 inability of observers to convey by description an idea of 

 the appearance of these iridescent objects, and having myself, 

 for many years, constantly experienced the same difficulty, 

 I have been led to adopt a method for the examination of 

 such objects, which, whilst extremely simple and available 

 in its application, yields unvarying results with different 

 observers, results, moreover, which admit of the simplest 

 description. 



Before describing this method, I may say that long 

 experience in the examination of iridescent objects, has 



