326 Dr BREWSTER on certain new Phenomena of Colour 



indurated matter, but, when minutely examined, this resemblance 

 disappears. The spaces between the individual patches are in 

 almost every case dark, like the adjacent felspar ; and when the 

 microscope is capable of separating the individual patches, it be- 

 comes quite obvious, that, if they are grains of indurated matter, 

 they are not disseminated through an empty cavity, but are im- 

 bedded in the felspar. We have no hesitation, therefore, in con- 

 cluding, that all these little patches and specks of colour are 

 empty cavities, like the large parallelograms, for the intensity of 

 the light reflected from the small patches in Nos. 3, & 4. of Fig. 2, 

 is the same as that reflected from the parallelograms. This light, 

 indeed, is so strong, that nothing but a metallic substance filling 

 the cavities, and in optical contact with their sides, could reflect 

 it. If this were the fact, the analysis of the mineral could not 

 fail to exhibit it, and I am not aware that any metallic ingre- 

 dient, except titanium, has been detected in felspar. M. PES- 

 CHIER has announced this fact, but whether it was found in com- 

 mon Felspar or Labradorite, I have not the means of ascertain- 

 ing. Professor ROSE of Berlin, however, who carefully analysed 

 the Labradorite of various localities, has not been able to disco- 

 ver any such ingredient. But even if titanium were a constant 

 element of Labradorite, the parallelograms could not contain 

 that metal ; for I have ascertained that titanium in optical con- 

 tact with mica reflects much less light than the parallelograms ; 

 and since mica has a refractive power greatly inferior to felspar, 

 titanium in optical contact with felspar, will reflect much less 

 light than in contact with mica, and consequently much less light 

 than the parallelograms. 



Having thus determined that all the colours under our consi- 

 deration are those of thin plates produced by minute cavities 

 within the mineral, varying in magnitude from the 40th of an inch 

 down to the most minute speck which the microscope can descry, 

 we are entitled to refer the other phenomena of colour in the 



