1892.] THE MICEOSCOPE. 127 



the same purpose may be effected by lengthening the tube of your 

 microscope. If, on the contrary, you want to correct for a thicker 

 cover-glass, set the systems closer or make your tube shorter. 



I. Thicker cover, longer tube, and opening systems tend to 

 over-correction . 



II. Closing systems, thinner cover, and shorter tube tend to 

 under-correction . 



For the recognition of under- or over-correction by the appear- 

 ance of the object, the writer has found the method of E. Gund- 

 lach to be of great practical value ; and he would urge careful 

 practical study of these appearances as affording a guide to the 

 kind of correction needed, whether '* under " or "• over." 



To illustrate the practical use of the above, suppose, for exam- 

 ple, a condition of under-correction of "general spherical aber- 

 ration ; " then either of the conditions named will afford counter- 

 action or correction, /. ^., either thicker cover-sj-lass, long-er tube. 

 or opening systems of objective, whichever maybe most conven- 

 ient or practicable. It is, of course, in many cases impossible 

 to alter the distance of lens-systems, owing to the objective being 

 in a fixed mount, not adjustable ; in such cases one of the other 

 correctives may be applied. — E. Pejinock in ^uee7i' s Pocket 

 Catalog7ie. 



Using Oil-immersion Objectives. — In using these objec- 

 tives cleanliness is important. Only a small quantity of the 

 immersion fluid (specially prepared cedar oil) should be used, 

 and it should be wiped off as soon as possible when done using. 



To remove the oil, blotting-paper should be used, and then, 

 breathing on the front lens, wipe it lightly with a piece of clean, 

 soft linen. 



In order to keep the immersion fluid unchanged it should not 

 be exposed to the air for any length of time, as exposure to the 

 air will thicken it, and so alter the refractive index. — E. Pen- 

 nock. 



Preparing Sponges. — W. R. Melly reported to the British 

 Association (1S91) the following solution for killing sponges ex- 

 tended : 



96 per cent, methyl alcohol ro parts 



Salt water 90 " 



Natrium chloride tV " 



He places the specimen in a glass jar (2x1 inch) filled with 

 water. Then add the solution, putting in one drop per minute ; 

 if not retracted after 45 minutes, pour on quickly some hot sub- 

 limate ; in this way some specimens are secured half-retracted. 

 To preserve specimens for sections put them in i per cent, os- 

 mic acid (for 3 minutes), and then successively in 5, 10, 20, 30 

 jDer cent, alcohol up to 90 per cent. ; harden in absolute alcohol 

 and imbed in parafline ; stain with borax-carmine and haema- 

 toxylin. 



