1882.] 



MICROSCOPICAL JOURNAL. 



189 



taining the injection-mass should be 

 attached to the nozzle stop-cock, 

 which fits the canulse to be used in 

 the injection. 



Now, we shall suppose that the 

 mercury has been placed in the sta- 

 tionary filtering funnel, and this lat- 

 ter firmly fixed a little above the mid- 

 dle point of one of the iron rods. 



The stop-cock of this funnel should 

 be turned so as to prevent the mer- 

 cury flov/ing from it, then the three- 

 way stop-cock connected with it 

 should be turned so that the air may 

 have access to surface of the mercury, 

 and the other three-way stop-cock 

 turned so that the air from the fun- 

 nel with which it is connected will 

 flow directly through the glass tube 

 to the Woulffs-bottle. 



Then if the second filtering funnel 

 be placed in a lower position than the 

 first above described, and clamped 

 securely, the machine is ready for 

 use. If the mercury should then be 

 turned on it will force the air out of 

 the lower filtering funnel, and this 

 air will then act upon the injection- 

 mass, forcing it along in proportion 

 to the difference in level of mercury, 

 or the amount of mercury turned on. 



If a larger quantity of injection- 

 mass is required than can be dis- 

 charged from the Woulffs-bottle by 

 once emptying one funnel into the 

 other, after the flow of mercury has 

 been stopped by turning the stop- 

 cock of the lower funnel an addi- 

 tional quantity of the fluid can be 

 poured through the central mouth of 

 the Woulffs-bottle. 



The positions of the three-way 

 stop-cocks should now be reversed to 

 permit the current of air to act from 

 the other funnel upon the injection 

 mass. 



Then the funnel, which is full of 

 mercury, should be raised to a posi- 

 tion on its rod as much above that of 

 the stationary funnel as it was below 

 it before, when it will, in its turn, be- 

 come the source of power when the 

 mercury is turned on. Thus the 

 operation may be proceeded with, 



and by repeating this simple process 

 an injection may be performed, last- 

 ing any reasonable time, with very 

 little trouble or interference on the 

 part of the operator, and, as far as 

 my experience goes, with great satis- 

 faction. 



It will be readily seen that abso- 

 lute constancy of pressure is not even 

 obtained in this instance, because the 

 shifting lines of level of the surface 

 of the mercury in the firmly fixed 

 funnels introduces a small error. 

 This might be overcome by suspend- 

 ing the funnels from springs, so ad- 

 justed that, as the mercury changed 

 its level a compensation would be 

 brought about by a change in the 

 position of the funnels ; but after 

 having tried the firmly fixed funnels, 

 and not having experienced the 

 slightest difficulty from this source of 

 error, I am inclined to believe that a 

 change would be an almost needless 

 refinement of accuracy, and, in addi- 

 tion, would somewhat doubt the pru- 

 dence of having such a weight of 

 mercury suspended in that manner. 



The piece of apparatus has been 

 thus described somewhat in the hope 

 that histologists will find in it an aid 

 in their investigations such as it has 

 been to myself. 

 Histological Laboratory, 



Princeton, N. J., 



The Physiology of Tariable Ap- 



pareiit Magnification by 



the Microscope.* 



BY W. LECONTE STEVENS. 



In estimating the size of an ob- 

 ject viewed in the microscope, it is 

 commonly assumed that the image is 

 seen as if at the distance of easiest 

 vision, which is taken to be ten 

 inches. The invalidity of this latter 

 assumption is strikingly shown in the 

 table of estimates exhibited and dis- 

 cussed by Prof. Brewer, in his recent 



* Remarks made before the section of 

 Physics of the A. A. A. S., with additions 

 by the author. 



