MI ACCOfTRFMENT FOR THE Fl) 33 



geology high powers are not required, consequently a 

 roscope is sufficient. Two powers 1} tnd 

 :i focal length arc extremely useful, and for the 

 rements of work in the field are quite adequate. 

 An instrument with fairly good glasses of these powers, 

 magnifying from 30 to atx>ut 300 diameters, according to 

 the arrangement of object-glasses and eye-pieces, may 

 be had of some London makers for Ji,$. If more pre- 

 cise and detailed petrographical determination is desired, 

 scope that the geologist or petrographer 

 can obtain is that devised by V k, and manu- 



d by Swift and Son, Tottenham Court Road, 

 London. The great advantage of this instrument is its 

 avoidance of the trouble of "centering," and the con- 

 sequent rapidity and ease with which it can be adjusted. 

 The price of the stand, without objectives, is ji&. 



It is sometimes of service, when working in a district 

 where microscopic rock-sections are required, to carry a 

 small collection of microscopic slices of selected or 

 typical minerals or rocks, for purposes of comparison. 

 A series of fifty or one hundred slices can be packed in 

 a box a few inches square. 1 Much assistance may be 

 derived from the study of a collection of minute grains 

 of minerals, obtained by reducing rocks to powder (see 



/Mfe* p. 236), 



1 Typical series of this kind may be had from Fues*. of Berlin, or 

 from Butler, London ; or Turnbull and Co., Edinburgh. 



