ARTIFICIAL LIGHT. 195 



the instrument shall only be illuminated sufficiently 

 to enable you to see your object distinctly. Now, 

 keeping things thus, revolve the collar, and notice the 

 effect on the illumination, and thus, as in the case 

 already presented, you have the means of judging as 

 to the aperture of the objective. And as an example 

 I now relate a bit of experience not twenty-four hours 

 old : We have just had in hand an objective claiming to 

 have high balsam angle, and we desire to know some- 

 thing about it. First, we look to its working distance 

 and find that it will work through covers one-fiftieth of 

 an inch thick, its distance is therefore ample. Applying 

 the Wenham " reflex " we test as to aperture, and pre- 

 cisely as has been above described, thereby learning 

 that its greatest angle occurs when the systems are at 

 "closed." We find, too, that as the collar is revolved 

 towards the " open point," the angle goes down rapidly. 

 We therefore conclude that to work this glass at its 

 maximum performance it will be necessary to use covers 

 thick enough to cause the objective to "correct" at or 

 near " closed.' 5 It will take but a moment to try the 

 actual experiment, aud to see if theory holds good in 

 practice. For this 'purpose we place the "reflex" in 

 position and the No. 20 of the balsam Moller plate on 

 the stage, making immersion contact with water. Next, 

 we attempt the resolution of the shell, and with the 

 best manipulations at our command succeed in getting 

 but a tolerable show of the striae. I have the blue field 

 in hand, with the lamp at the extremest point to the 

 left: the best display being thus obtained, as the lamp 



