202 Dk. a. Hodgkinson on 



probably caused by the grinding, thus leaving the natural 

 colours of mother of pearl due to thin plates alone. Again,, 

 another common mode of colour-production, absorption, 

 may co-exist with that of thin plates, constituting a method 

 of colour-production not, so far as I am aware, before 

 described. From the mode of formation of this class of 

 colours I shall refer to them as the Colours of TJiin 

 Absorption Plates^ and under this title I have devoted a 

 section to their consideration. This mode of colour-pro- 

 duction is interesting, as affording an explanation of the 

 reflected and transmitted colours of metals, aniline colours 

 and iridescent crystals of permanganate of potash, and some 

 other almost opaque bodies. 



Finding, as a result of experiment, that these two 

 colour-producing structures — fine lines and thin plates — 

 when in their typically perfect condition, and when 

 examined in a suitable way, produced optical effects 

 peculiar to, and therefore characteristic of, such struc- 

 tures, and noting that even when the structures were 

 not typically perfect, proportionately characteristic results 

 were obtained, I was led to see that there are other indi- 

 cations of structure than mere image formation ; that 

 there are, in fact, two ways in which minute structure may 

 reveal itself by the agency of light ; one in which the 

 illuminating light is so refracted or reflected as to admit of 

 the formation by means of one or more lenses of an appre- 

 ciable image on the retina. This, which may be termed 

 the direct method, is what occurs in ordinary microscopic 

 observation when the instrument is used as a mere magni- 

 fying appliance, and in this instance we have to do 

 with an image of the object identical in appearance and 

 differing from such object only in size. In the second or 

 indirect method the structure so materially modifies the 

 light as to reveal itself, not in the form of an image or 

 replica of itself, but b}- the production of some other optical 



