586 Transactions of the Society. 



of the slide, so as to be connected with a slender elastic tube through 

 which gases or vapours may be projected, and which, after traversing 

 the annulus, escape by the other tube. A ring of olive oil is set 

 around the annulus upon the surface of the slide, and the cover 

 containing on its centre the drop of liquid to be examined is then 

 so placed over the annulus that a film of fluid less than a quarter- 

 inch in diameter lies between the top of the central paraboloid and 

 the cover, while the margin is sealed by the oil. Thus the object 

 may be examined, firstly by itself, and afterwards while various 

 gases are passed through the surrounding annular space, and the 

 changes produced, say on blood-disks by dry or moist air, alcohol 

 vapour, ammonia, acetic acid, carbonic acid, &c., can be watched 

 and repeated at pleasure. 



If light be thrown into the central area from beneath by means 

 of a two-inch objective, the object is seen negatively upon a bright 

 field, while if the condenser be decentered the light is thrown 

 upon the parabolic surface and is totally reflected into the object 

 at such angle as to give a positive image upon a black background 

 under a dry eighth or sixteenth, as with the immersion paraboloid. 

 The glass tubes may be cemented into the groove by means of dough, 

 putty, jDlaster of paris, shellac, &c. If very hot sealing-wax be 

 used the slide is apt to crack, unless previously heated in water. 

 Diaphragms of black paper or tin-foil may be gummed on to the 

 lower sm-face of the slide, so as to stop out the central area when 

 black-field observations are wanted. Both as a gas slide and as a 

 simple form of the immersion paraboloid, it works conveniently and 

 efficiently. 



