418 Proceedings of the Royal Physical Society. 



line of axial movement of this instrument. Nor is there 

 any difficulty in working in broad daylight, seeing that the 

 orange-coloured light of the gas flame is seen to be quite 

 distinct from the whiter light of day. There is no difficulty 

 in adjusting the steel pointer to any radial zone on the map 

 corresponding to any known face on the crystal; and, of 

 course, the movement which brings the next face into the 

 reflecting position moves also the pointer to the radial zone, 

 in which lies the second face whose position has to be 

 determined. This somewhat rough, but very simple and 

 effective instrument, usually gives most satisfactory results. 

 Instead of costing many pounds, its total cost need not 

 exceed that of a few pence, and a man must be a very poor 

 mechanic indeed if he cannot construct one like it for him- 

 self in an hour or so. Of course, it is not meant for original 

 research, nor is it of much use where the species of mineral 

 under observation does not happen to be known. Further- 

 more, it is of not much use without a stereogram. All the 

 same, I have done much work with it, as any visitor to the 

 Scottish Mineral Collection can at once see. Well on to a 

 thousand crystals, of many different species, have had their 

 forms determined by its aid ; and the drawings made after 

 these determinations are placed in contiguity to the actual 

 specimens, which are all properly indicated, so that visitors 

 may, if they desire to do so, examine both drawing and 

 original at any time, and decide for themselves how far the 

 two agree. 



As a worker grows familiar with the physical charac- 

 teristics of the different forms on each species of mineral, the 

 need for examination by goniometric methods gradually 

 becomes less, and may, with long experience, cease to exist. 

 I know that this statement will be received with incredulity 

 by many persons ; nevertheless, it is a fact. 



Methods of Drawing Crystals. — In dealing with museum- 

 specimens, every curator must know that, in many cases, he 

 cannot, and must not, damage a valuable mineral by chip- 

 ping a crystal off its matrix in order to measure it. If that 

 crystal is to be determined at all, it is obvious, in such a case. 



