of the Hyalosiderite. 1 89 



on P of the annexed figure) amounted to rather more than 

 130°." 



Now I have constantly found by means of the reflective 

 goniometer, that the former of these angles is about 2 degrees 

 less, the latter about 2 degrees greater, than Dr. Walchner's 

 determination. I forbear, nevertheless, from annexing re- 

 marks, which naturally ainse from a consideration of the pre- 

 ceding extracts from Dr. Walchner's communication ; and 

 should have been content with simply noticing the differences 

 between his measurements and those afforded by the reflective 

 goniometer, which alone is adapted for crystals so minute as 

 those of the hyalosiderite (for no one of mine exceeds l-20th 

 of an inch in any direction), if I could have persuaded myself 

 that some ill-founded prejudice against that admirable instru- 

 ment, does not exist on the European continent generally, 

 notwithstanding the many errors it has served to correct, the 

 nice differences it has detected, and the ease with which it 

 may be used. Rarely does a foreign mineralogist visit this 

 country, who is acquainted with it except in theory: and I 

 believe that in every foi'eign work on mineralogy, it is figured, 

 not with the moveable pin at right angles, but horizontally, in 

 continuation as it may be termed, with the axis, in which po- 

 sition it is almost useless. These observations are penned in 

 the hope that they may meet the eye of Dr. Walchner, and 

 serve to induce him to prove the superiority of the I'eflective 

 goniometer, and consequently the futility of the prejudice 

 against it, if in reality it exists. 



We know, however, that every measurement, by whatso- 

 ever means it may be made, is, from the natural imperfection 

 of crystalline planes, rarely to be estimated but as an approxi- 

 mation ; but it is also known from experience, that the I'eflec- 

 tive goniometer is most constant in its results, and the only 

 one adapted for the measurement of small crystals, which com- 

 monly are by much the most accurate ; for the results obtained 

 by measuring them, agree much better with each other, than 

 those obtained from large crystals. 



There is at least one plane on the crystals of hyalosiderite 

 which has not been observed by Dr. Walchner; and which, as 

 it appears to me, is a very important one : it is the plane M 

 of the following figure : most of the crystals in my possession 

 show it, and on some of them it is comparatively large. Its 

 presence has induced me to assume the primary form to be a 

 right rhombic prism of about 105° and 75°. I say, about, 

 because, owing perhaps to the brittleness of these crystals in- 

 ternally, I have not been able to detect with certainty a cleav- 

 age in any direction, and because we can only rely upon se- 

 veral 



