198 THE MICROSCOPE. [September, 



must be placed above the objective, anywhere in the optical axis. 

 One of the prisms must be arranged to revolve, and for conven- 

 ience the polarizer is so mounted. At certain relative angular 

 positions of the prisms the polarized light v^'ill display colors in 

 a suitable transparent object. Good practical directions for the 

 use of the polariscope may be found in The Microscope and His- 

 tologv. part I. by S. H. Gage, printed by Andrus & Church, 

 Ithaca. X. Y. A good description of the apparatus may be 

 found in Practical Microscopy, by Geo. E. Davis, published 

 by Lippincott Co., Philadelphia, Pa. 



6S. I/ozv shall I prepai'e the leaf of lettuce^ to shozu the 

 laticiform tubes? 



Make a long section of the midrib ; boil in a weak potash 

 solution until tender ; tease out the vessels ; float them on a slip, 

 and examine in glycerine. They will not be permanent, but 

 may last several months. 



69. How shall I nioiint cross sections of steins of apple ^ pear^ 

 currant^ etc.^ as opaque objects? — Amateur . 



Dry the stems ; bore a hole in a block of wood, j^ack in the 

 stem snugly ; use a sharp smoothing plane to shave off the top 

 of the block including the stems ; pick out the sections and 

 mount them dry in a shallow cell formed by a ring of varnish. 

 Keep the sections dry. 



70. Why does 7nica show prisTuatlc colors like Newton^s 

 rings y — Polariscope. 



Mica is composed of thin plates or leaves, easily separable by 

 splitting or bending. Loosened or partly separated plates will 

 show the colored rings, produced by diffraction of light. 



71. Da7ia says water is a mineral. If so, is it proper to 

 say '''' miller al water''"' ? — E. C. Hoyt. 



Mineral water is a technical or trade term applied to a natural 

 water containing soluble mineral substances in appreciable quan- 

 tities, in contradistinction to a good drinking water, which 

 should not contain more than 10 grains of solids to the gallon. 



72. Is there life in all water ? Is there more life in a drop 

 of rain-water than in artesian well water ? — E. C. Hoyt. 



Bacteria and other fungi will be found in all water coming in 

 contact with our atmosphere. Rain-water collected during the 

 latter part of a protracted storm will be nearly devoid of life. 

 Artesian well water, being in contact with the earth, will con- 

 tain more germs than rain-water. 



73. How are insect eggs preserved? — Student. 



They are best mounted dry and viewed with a surface illumi- 

 nation. 



