November 24, 1892] 



NATURE 



93 



beautiful effect." Most observers, in describing the colours of 

 iridescent bodies, do so by attempting to depict the varied 

 effects produced by casually changing the position of 

 the object in relation to the light, omitting to mention the 

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

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

 duced by perpendicular or oblique illumination. Here is a 

 description of the tufted neck humming bird, Trochilus ornatus, 

 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 descriptions 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 describing 

 this humming bird, Chryslanipis mosquitus, remarks: — " It is 

 impossible to convey by words the idea of these tints, and 

 having mentioned those substances to which they approach 

 nearest, iinagination 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 iri- 

 descent objects, and havtng 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 ob-ervers, results, moreover, which admit 

 of the simplest description. 



Before describing this method, 1 may say that long experience 

 in the examination of iridescent objects has proved to me that, 

 almost without exception, the colours of natural iridescent 

 objects are due to interference produced by thin plates. In 

 order, therefore, to render clear the principles on which the 

 method I propose is founded, I will briefly refer to certain fun- 

 damental facts in connection with colour production by thin 

 plates, and for this purpose will select a thin film of mica, which 

 with light at perpendicular incidence, appears red, iridescent 

 red. If, now, this plate be inclined so that the light falls on j 

 it at a more oblique angle, it is, of course, reflected at the same ^ 

 angle, and now appears orange, and if the plate be still further | 

 inclined, the reflected light appears yellow, then yellowish 

 green, green, and bluish green, and if the light were not too 

 copiously reflected from the first surface to allow of perceptible 

 interference by further inclination of the plate, all the colours 

 of the spectrum in their proper sequence might be observed. ■ 

 The same results, but much more vividly, may be seen in these [ 

 crystals of chlorate of potash. Thus, we see that by rendering 

 the incident light more and more oblique, the reflected light 

 changes from a lower to a higher tint, that is, from the red | 

 towards the violet end of the spectrum. And this is what j 

 occurs in the case of all iridescent bodies, as the incident ! 

 light becomes more oblique the colour changes to the tint above ■ 

 it in the spectral order, so that, if we know what colour any 

 such object appears when seen at a certain angle, we can infer ; 

 what colour it will change to on varying the incidence. This \ 

 beetle {Sagra purpurea), for instance, is red at perpendicular | 

 incidence, it will, therefore, appear orange yellow and green 

 when examined by successively increased obliquity of light. 

 And the same is true of all other iridescent red objects. If j 

 the object at perpendicular incidence be green, as in the case ! 

 of this beetle (Bupristis), it will become blue and then violet \ 

 as the incidence is increased. We thus see that an iridescent j 

 object varies in colour, simply because it is examined by light i 

 incident, and therefore reflected, at different angles. Thus, j 

 different observers see the same iridescent object of a different 

 colour, when they view it illuminated by light at a different 

 angle of incidence. If, however, the object is seen by all at 

 the same angle of the incident light it will present the same 

 colour, and this is, in fact, what the method I propose ensures, 

 i.e., that iridescent objects shall always be seen by light at one 

 and the same angle of incidence. The angle I select is one of 

 90, so that the incidence and reflection are normal or perpen- 

 dicular to the reflecting surface. By selecting this angle all 



NO. I 204, VOL. 47] 



trouble of measuring angles is avoided, since we know that the- 

 incidence is perpendicular when it coincides with reflection. 

 Now, the reflected light may be made to coincide with the 

 incident light by reflecting it on to the object by means of a 

 mirror, and so adjusting the object that the light reflected from 

 it passes to the eye through a perforation in the mirror. When 

 examined in this way iridescent objects are marvelloisly altered 

 in appearance, their changing colours are replaced by one fixed 

 tint, visible only in one position, a fact which serves at once 

 to distinguish them from bodies coloured by absorption, which 

 ! remain coloured whatever the relation to the incident light. 

 Such methods of examining bodies scarcely takes more time 

 than by the eye alone. The mirror may be attached to a 

 spectacle frame so as to leave both hands free, such as the one 

 I show, or may hi a, simple hand mirror. For objects too 

 small to be seen by the unaided eye, I have so arranged the 

 microscope that light is made to pass down the tube of the instru- 

 ment, through the object glass on to the objects, and by a 

 special arrangement, so adjusted the position of the object that 

 ' the light is reflected back again through the instrument to the 

 I eye. The method is thus available for macroscopic as well as 

 I microscopic objects. 



I To illustrate the practical value of this plan of examination, 

 j I have here a few objects exhibiting iridescent colours, which, 

 by trial, will be found to give the following results : — 



The crest of this humming bird, Chrysolampis mosquitus, 

 j which, to the unaided eye, appears resplendent with all shades 

 ! of red, orange, yellow, or green, according to the angle of 

 the incident light, appears, when examined by the mirror, of 

 one unvarying red tint, disappearing when the object is moved 

 I but absolutely unchanging in tint. Such an object, therefore, 

 : I should describe as " iridescent red "; all else regarding its 

 colour may be inferred. Again, the breast, or gorget, ot the 

 same bird reflects all shades of orange, yellow, or green to the 

 eye alone ; with the mirror it is seen of a deep orange, which, 

 as before, is unchanged in tints by any variation in position. 

 Such an object I would describe as "iridescent orange." The 

 gorget of another humming bird, Calliphlox amethysiitta, to the 

 eye alone appears crimson, orange, yellow, or green : with the 

 mirror it is iridescent crimson only, spectroscopically a red of 

 the 2nd order. Amongst insects, instances of iridescent species 

 are numberless, the results of examination are just the same as 

 in other iridescent bodies. This butterfly, Morpho, to the eye 

 alone appears either greenish-blue, blue, or violet, as its 

 inclination to the light varies ; examined with the mirror it 

 appears green, and should be described as iridescent green, or 

 iridescent bluish-green. This beetle, Foroplettra bacca, appears 

 any shade of red, yellow, or green to the eye alone ; with the 

 mirror only iridescent red. In this extraordinary beetle, 

 Chrysochroa ftilminans, we have all the colours of the spectrum 

 in their natural sequence, beginning with red at the tip of the 

 wing case, and ending with violet higher up the elytron. These 

 colours vary in an indescribable manner when attentively 

 examined at different angles of incident light with the eye 

 alone ; with the mirror the wing cases are seen to be coloured 

 successively from base to tip iridescent green, yellow, orange, 

 and red, and these tints remain unaltered by change of position 

 of the object. This piece of /^a/jW?V shell exhibits indescriba- 

 ble changes of colour with every movement, but the difficulty 

 of description, though by no means removed, is immeasurably 

 lessened by the use of the mirror. And the same with this 

 specimen of iridescent iron ore, its colours, which vary to the un- 

 aided eye, remain unchanged when examined by the mirror. 

 To simplify the description of iridescent objects, therefore, I 

 would advocate the above method, and would describe the 

 result of such examination by recording the colour observed 

 by aid of the mirror, and prefixing the term "iridescent" to 

 express the changing properties of the colour. Bearing in 

 mind the unvarying nature of these changes, a far clearer idea 

 may be formed of the appearance of these objects than from 

 any attempted description of what is admittedly indescribable. 

 Time and space are also economized by the omission of lengthy 

 descriptions. The accuracy, and, therefore, the value of any 

 description of colour, is always enhanced by mapping its spec- 

 trum ; more especially is this true in the case of iridescent 

 colours. This is easily done, and by applying such map to a 

 spectral chart, the order of the colour, and therefore its tint, is 

 apparent. In examining many objects, chiefly birds or insects, 

 by means of the mirror as above described, apparent exceptions 

 are repeatedly met with to the fact stated above, that the colour 



