268 



Prof, Miller on the Form of Buttle. 



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 llutile. By W. H. Miller, 



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

 bridge. 



nPHE 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", p" 1 

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

 Fig. 1. Fig. 2. 



defined. The observed values of p' p m 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 



