186 



On an Improved apparatus for the preparation of Rock 

 Sections for Microscopical examination. By James B. 

 Jordan. 



{Read February 26th, 1869.) 



To describe with accuracy the mineral composition of a given rock 

 is often a task of considerable difficulty, especially if the individual 

 minerals occur only in minute crystals or in granular masses, and 

 are so intimately associated as to show to the naked eye only a 

 fine grained appearance. Such rocks nevertheless frequently fall 

 under the notice of the geologist, who of course gladly avails him- 

 self of any aid — whether chemical, physical, or mechanical — which 

 may assist him in his examination. Among these aids there is none 

 more important than that offered by the microscope. By studying 

 sections of sufficient thinness to admit of examination by trans- 

 mitted light, considerable insight is gained as to the character of 

 the component minerals of the rock and their mode of association ; 

 whilst the value of such observations may be increased by intro- 

 ducing the use of polarized light. 



The subject of thus preparing thin sections of hard material 

 has been fully treated by Mr. Sorby, and also by Mr. David Forbes. 

 At the request, however, of our secretary, I venture to bring it 

 before you this evening, for the benefit of those who are interested 

 in this branch of microscopical science. 



Having had occasion to cut and grind a series of geological 

 specimens of rocks, fossils, and other mineral structures, I could 

 not find that there was any machine to be purchased suitable for 

 the purpose ; I was therefore led to design an appaiatus for myself, 

 which I have had in operation for about twelve months. As will be 

 seen from the diagram (on next page), this machine is in some respects 

 similar to that described by Mr. Butterworth in the January number 

 of Science Gossip. It consists of a wooden framework, a a, made of 

 beech-wood, and supporting a crank axle and driving-wheel, two 

 feet diameter ; the top-part of this frame consists of two cross- 

 pieces, a' fixed about an inch apart, as in the bed of an ordinary 



