268 



Prof. Miller on the Form ofRutilc. 



alone to an explanation of the peculiar mode of operation ex- 

 hibited by the hydriodic salts on different preparations of silver 

 under the influence of light, have thus opened a new and 

 unexpected field of interesting inquiry, which may possibly 

 end in the establishment of the new art of THERMOGRAPHY. 



I shall pursue this subject with the same interest by which 

 I have been led forward in my inquiries on Photography since 

 the publication of Mr. Talbot's processes and those of Da- 

 guerre. For a few interesting applications and curious dis- 

 coveries which I have made I merit not, nor do I seek praise. 

 To every inquirer there is a mine of discovery, of which the 

 few specimens I have gathered on the surface will, I trust, 

 show the richness of the yet buried treasure. 



I remain, Gentlemen, yours, &c. 

 Devonport, July 4, 1840. ROBERT HUNT. 



XXXIX. On the Form of Untile. By W. H. MILLER, 

 Esq.) Professor of Mineralogy in the University of Cam- 

 bridge. 



HPHE following values of the angles between normals to 

 * the faces of rutile were obtained from two extremely 

 perfect crystals, for which I am indebted to Mr. Brooke. 

 The instrument used as a goniometer was a twelve-inch theo- 

 dolite. The coincidences of the signals were observed with 

 a telescope having a power of about twelve. Each result is 

 the mean of two observations made with the signals inter- 

 changed, in order to eliminate the error arising from the im- 

 perfect centring of the crystal. In one the faces p 11 , p'" 

 (fig. 1.) gave double images, those in p 1 " being close and ill- 

 Fig. 1. Fig. 2. 



defined. The observed values of p' p" 1 were 65 33' 0" 30" 

 8"; those of pp" 65 32' 22", or 65 34' 26", according as 

 one or the other of the images was made to coincide with the 

 signal seen by direct vision. In the second and more perfect 



