76 THE MICROSCOPE. 



but the screw collar is a necessity. Objects only 140,000 to the 

 inch, or more or less, are difficult to focus, and cover glasses and 

 other conditions may make it impossible to focus them. In lower 

 powers, unless the angular aperture is very high, the screw collar is 

 not a necessity. 



In purchasing a microscope, one of American make is generally 

 preferable. The stands of medium size and weight, made by 

 Bausch & Lomb, of Rochester, N. Y., or Tolles, of Boston, are 

 excelled by no makers; neither are their objectives. My own stand 

 was made by Tolles, after plans devised by Dr. Geo. E. Blackham, 

 and the low power objectives and one-eighth of 180° were made by 

 Bausch &: Lomb. No doctor can get any good out of a poor micro- 

 scope, for the reason that toys do not even amuse grown up people 

 for any length of time. — Dr. R. J.. Curtis, in the Peoria Medical 

 Monthly. 



The Physiology of the Oyster, Ostrea Virginiana. 

 By Prof. Samuel Lockwood, Ph. D., Secretary of the New Jersey 

 State Microscopical Society. 



1. The relative parts.— After saying that, in deference to his 

 audience, he would avoid technicalities as much as possible, taking 

 from the table a round clam, or quahaug, [M. mercenaria,) the pro- 

 fessor said he would use that shell for the purpose of introducing 

 the oyster. The round clam is somewhat of a traveler, ploughing 

 its way through the mud with its great pedal disc, which is thrust 

 from the opened shell. Let me put the shell in its natural position 

 for such work. Here it is — the edge downward and the thick, or 

 hinged, side uppermost. You notice I have the two beaks turned 

 from me. The ploughing foot would, if protruded, be towards me. 

 The side of the shell at my right hand is the position of ihe right 

 side of the animal, that to my left is its left side; the side uppermost 

 is its dorsal side, and the one below, that is, the one which enters 

 the mud, is the ventral side. But we have to view the oyster dif- 

 ferently. Except in the embryo state it is no traveler. Once settled 

 it becomes a fixture for life; and in its normal position it lies on one 

 side. The two valves of this clam are equal, but those of the oyster 

 are very unequal. One valve is almost flat. It is on this that the 

 caterer serves the mollusc when taken on "the half shell." Th^ 

 other valve is rounded and deeply dished, and much the heavier of 



