Dr Heineken on the Birds of Madeira. 229 



brought into optical contact, so that we ought to have had 

 orders of colours corresponding to a film of twice the thick- 

 ness. 



But even if such a film could be supposed to exist invisibly 

 on the glass, it could not afford any explanation of the splen- 

 didcolours which are exhibited when the solid is a cryst allized 

 mineral, and where its tint is related to its axis of double re- 

 fraction. That some unrecognized physical principle is the 

 cause of all these phenomena, will appear still more probable 

 when I submit to the Society a paper on the very same pe- 

 riods of colour produced at similar angles of incidence, by the 

 surfaces of metals and transparent solids when acting singly 

 upon light. 



The action of the surfaces of crystallized bodies presents 

 many remarkable phenomena, in the investigation of which I 

 have been long occupied. The results to which I have been 

 led will form the subject of two communications. The first 

 will treat of the action of the surfaces of bodies as an universal 

 mineralogical character, with the description of a lithoscope for 

 discriminating minerals. The second will contain an inquiry 

 into the influence of the doubly refracting forces upon the or 

 dinary forces which reflect and polarise light at the surfaces of 

 bodies. My early experiments on this subject are recorded in 

 the Phil Trans, for 1819, but I have resumed the inquiry, 

 and have obtained results of considerable interest *. 



Allerly, February % 1829.^ 



Art. IV. — Notice of some of the Birds of Madeira, By 

 C. Heineken, M. D. Communicated by the Author. 



Sir, 

 If you can find room for the following notice of two or 

 three of our birds, I shall feel obliged by its insertion ; and 

 should they prove to be well known, I can only offer the iew 

 particulars of their habits, &c. which are added, as a compen- 



* From the Phil. Trans. 1829, Part 1, p. 18T 



