ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



881 



slide was then moved down till the quartz came into view, and the 

 position of the slide again read off on the scales. The object of 



Fig. 212. 



Adularia 



An occasional spicule .... 



Middle of zone of calcareous spicule . 

 Beginning of same zone 



Quartz and perforate foraminifera . 



Perforate foraminifera (lowest lyin; 

 examples) 



Calclte and Milliola 



Lowest lying Jililliola 



2-72 2-7 



a. Axis of distances. 



2-67 2-65 2-63 262 



b. Axis of specific gravities. 



having two scales is obviously to ensure parallelism in the movements 

 of the glass slide. 



The specific gravities and distances being indicated on the rect- 

 angular axes, one constructs a curve which gives the change in 

 density from one mineral to another in the tube. 



The height of the zone of spicules being now indicated on the axis 

 of distances, a line is drawn parallel to the other axis through it ; 

 from the point where it cuts the curve a perpendicular is let fall on 

 the axis of specific gravities, and the point where it meets the axis 

 gives the specific gravity. In this way the specific gravity of the 

 spicules was determined to be from 2 '61 to 2 "63. They are plainly, 

 therefore, not aragonite, and, arguing from the specific gravity alone, 

 probably consist of calcite. The slight difierence between it and 

 them in specific gravity is no doubt due to the presence of organic 

 matter ; for within, a minute canal, filled with some kind of organic 

 material, possibly epongin, occurs in the axis of the spicules ; and 

 without they arc surrounded by a thin sheath of a probably similar 

 material. Prof. Sollas finds by calculation that allowing for the 

 organic matter a specific gravity of 1*5, it would require to be 

 present to the extent of 6| per cent, to reduce the total specific 

 gravity of the spicules from 2 • 7 (supposing them to consist chiefly 

 of calcite) to 2 • 02, the density found. 



Keeping both Eyes open in Observation.* — Mr. E. M. Nelson 

 considers that the unused eye should be shut when the weaker light 

 is in the Microscope, both eyes being kept open only when the object 

 is in the stronger light. Thus by difiusod daylight the light in the 

 instrument is the weaker, and the other eye must bo shut. By 

 artificial light in a dark room both eyes can be kept open. " One 

 hour of steady hard work with the Microscope by diffused daylight 



V.np]- Mecli., xli. (1885) p. 523. 



