MINUTE STRUCTtJKE OF THE SPERMATOZOON OF NEWT. 339 



some,^ and sooner or later, to my experience, the staining runs, 

 and the preparation is spoiled. Solutions of acetate of potash or 

 chloride of calcium I have not found satisfactory ; the form 

 even of such resistant objects as bacteria, in some cases becoming 

 materially altered by these reagents. With Canada balsam, 

 even when dissolved in chloroform or turpentine, I have not 

 found the preparations fade, as has sometimes been said to be 

 the case, and as we should have expected ; nor, if they are suffi- 

 ciently washed in alcohol and passed through oil of cloves, 

 will they run ; the risk, however, both of fading and running 

 may be entirely pbviated by using benzine as a solvent for the 

 balsam, or by employing it undiluted and liquified by 

 warmth. 



In examining this structure I have employed the -Vth 

 homogeneous immersion of Messrs. Powell and Lealand, which 

 having the very high numerical aperture of 1-38° gives, with 

 admirable light and definition, an amplification of about 3400 

 diameters, with an eye-piece of | m. focal length ; the barb, 

 however, in a suitable preparation, may be readily seen and 

 examined with a good ^ th objective. Even with much lower 

 powers, as e.g. the -j^th P. and L. I have recognised it, 

 dependent however much upon the method of illumination 

 employed : for, as is generally recognised, good illumination 

 will show an object with a much lower power than is requisite 

 in the ordinary way. The best means of this as yet available 

 is the direct light of the flame of a paraffin lamp turned edge- 

 wise to the observer, whether with or without a substa»e con- 

 denser. This was recommended by Dr. L. Beale thirty years 

 ago, and is now again frequently adopted. Light reflected 

 from any mirror is in some way inferior to direct light, and 

 this not owing to the double reflecting surface of ordinary 

 mirrors, for I have tried them silvered on the upper surface 

 without any material advantage. 



' The metliod is, add an equal bulk oC glycerine to the aqueous solution of 

 the aniline dye used, stain somewhat more deeply than requisite, mount on 

 slide with cover glass in the staining fluid, which is to be gradually replaced 

 as the walcr evaporates by plain glycerine. 



