288 THE AMERICAN MONTHLY [December, 



beration " ; then either of the conditions named will afford 

 counter-action or correction, /. «?., either thicker cover-glass, 

 longer tube, or opening systems of objective, whichever may be 

 most convenient or practical. It is, of course, in many cases im- 

 possible to alter the distance of lens-systems, owing to the objec- 

 tive being in a lixed mount, not adjustable ; in such cases one of 

 the other correctives may be applied. — E. Pennock^ in Queen's 

 Pocket Catalogue. 



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

 tives cleanliness is important. Only a small quantity of the im- 

 mersion fluid (specially prepared cedar oil) should be used, and 

 it should be wiped oft' 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 iminersion 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. Pcnnock. 



Gulland's Method of Fixing Paraffin Sections to the 

 Slide. — After pointing out the difficulties arising from the use of 

 the albumen fixative, the author offers the following method : 



The tissue is imbedded in the usual manner. In trimming the 

 block for sectioning, care must be taken to see that the surface 

 meeting the razor is exactly parallel to the opposite surface ; these 

 surfaces are then coated with soft paraffin, and when this has 

 hardened are again trimmed square. The reason for this special 

 care is that any curve in the ribbon produced by neglect of this 

 precaution is accentuated by the later flattening of the sections. 

 When all the sections required have been cut, the ribbon is 

 divided into lengths corresponding to that of the cover-glass in 

 use. 



A flat glass dish filled with warm water is now provided ; the 

 temperature should never be high enough to melt the soft paraffin 

 holding the sections together. Short of this, however, the warmer 

 the water the more rapidly and completely are the sections 

 flattened. 



The ribbons are seized at one end with forceps while the other 

 end is gently lowered upon the surface of the warm water ; as 

 the sections flatten out they will move along the surface of the 

 water ; when the flattening is complete, the slide, carefully cleaned, 

 is immersed in the water. The ribbon is floated into its position 

 with a stiff' brush ; this process is repeated until the slide is full, 

 when it is set up on end until the water is thoroughly drained 

 off'. The slide is then transferred to the top of the imbedding 

 oven, where the temperature is a little under 50° C, and where, 

 consequently, the paraffin of the sections is not melted, though 

 the water rapidly evaporates. The slides are kept there with a 

 card-board cover over them to keep ofl^dust, until the evaporation 



