Miscellaneous, 79 



burrowing had but little effect ; and it was only after an hour's labour, 

 when the cavity had become large enough to allow the polype to 

 work in an upright position, and with the assistance of its whole 

 weight, that rapid progress was made. 



It is probably by this hydrostatic pressure that all burrowing soft- 

 bodied animals* having a water-circulation are enabled to work their 

 way into the ground. 



Application of Polarized Light to the Microscope, 

 By Dr. M. C. White. 



For conducting microscopic investigations with polarized light, it 

 is customary to employ two Nicol's prisms, one of which is placed 

 below the stage of the microscope, and the other just above the 

 object-glass. If the prism placed below the stage is large, a fine 

 illumination is obtained by light reflected from a concave mirror 

 placed below the prism, or by condensing the light, after it passes 

 the prism, by means of an achromatic condenser. But as the Nicol's 

 prism transmits only the extraordinary ray produced by double re- 

 fraction of the Iceland spar of which it is composed, one-half the 

 light is lost, and a powerful light is required to give adequate illumi- 

 nation. 



The Nicol's prism above the object-glass, used for an analyser, 

 requires to be large ; otherwise the lateral portions of each pencil 

 of light are cut off, the object seen in the microscope appears insuf- 

 ficiently illuminated, and the definition becomes very imperfect. 



On the other hand, if the Nicol's prism is very large, its length is 

 increased in the same proportion as the breadth, and considerable 

 aberration is produced by transmitting the pencil of light through so 

 great a thickness of Iceland spar. A lateral displacement takes place, 

 equal to about one-twelfth of the length of the Nicol's prism through 

 which the light is transmitted. 



These errors may be corrected, and the definition of delicate objects 

 greatly improved, by placing another prism, exactly like the first, 

 above it in the body of the microscope below the eye-piece, with its 

 plane of principal section rotated 180°. This corrects entirely the 

 lateral displacement. The second prism in the body of the micro- 

 scope may be simply a prism of Iceland spar of the same dimensions 

 as the Nicol's prism used as an analyser. This prism of Iceland 

 spar may be placed next above the object-glass, and the Nicol's prism, 

 if sufficiently large, just below the eye-piece, without altering the 

 effect. To show the important value of this compensating prism in 

 the body of the microscope, I will describe the results obtained with 

 this apparatus. 



With J. & W. Grunow's microscope. No. 4, and a polarizing prism 

 2^ inches in thickness beneath the stage, mounted to be used either 

 with or without an achromatic condenser, I placed in the body of the 

 microscope, above the object-glass, a Nicol's prism for an analyser, 

 measuring 2\ inches in thickness, and a prism of Iceland spar of the 

 same dimensions (obtained from Rossie, New York), in a reversed 



