134 



THE AMERICAN MONTHLY 



[July, 



He is pleased to be able to an- 

 nounce that he has already succeeded 

 in making some arrangements, while 

 in London, which will greatly enhance 

 the value of the Journal to many 

 readers. It is also not improbable 

 that after his return he will be able 

 to devote more time to the congenial 

 work of editing the Journal than has 

 been possible in the past. 



During his absence he has been 

 most kindly and efficiently aided in 

 preparing the paper by Mr. J no. L. 

 Wall, of New York, who has read all 

 the proofs, and arranged the matter 

 for each issue. If Mr. Wall had not 

 freely consented to do this work, which 

 has surely made considerable demands 

 upon his time, the Editor would have 

 experienced far more anxiety about 

 leaving than he did. Under these 

 circumstances the thanks of the read- 

 ers as well as of the Editor are due 

 to Mr. Wall for his assistance so freely 

 rendered. 



The Aperture Shutter. — There 

 has been considerable difference of 

 opinion among microscopists concern- 

 ing the value of the " aperture shut- 

 ter " as it is called, which is nothing 

 more than an adjustable diaphragm 

 fitted to the nose-piece of a micro- 

 scope just above the objective. It is 

 made by Mr. Charles Collins, of great 

 Portland Street, London, with a society 

 screw so as to be readily attached to 

 any microscope. Mr. Collins adver- 

 tises it " for giving penetration to ob- 

 jectives." Mr. George E. Davis pub- 

 lished an article some time ago in the 

 Northern Microscopist, in which he en- 

 deavored to prove that the use of the 

 aperture shutter does increase the 

 penetration of an objective, without 

 injuring its definition. The ground 

 thus taken by the author, which was 

 well sustained by the engravings of 

 two photographs of polycystina, taken 

 by an objective with its full aperture 

 and when its aperture had been re- 

 duced by means of the shutter, was 

 supposed to be in direct opposition to 



the results of Prof. Abbe's calcula- 

 tions. Prof. Abbe had said that the 

 use of diaphragms above an objective 

 to reduce the angular aperture would 

 destroy the perfection of the definition. 

 Mr. Davis endeavored to prove that 

 it was not so, and he certainly suc- 

 ceeded in doing so, in the particular 

 case illustrated in his article. 



We do not propose to enter into the 

 theory of the subject, but the question 

 presented is one of no little practical 

 importance. It is probable that both 

 Prof. Abbe and Mr. Davis are right, 

 for we believe that while Mr. Davis 

 has been experimenting with lenses of 

 low power. Prof. Abbe's results applied 

 to objectives of high power, in which 

 the conditions are quite different. 



The practical question that inter- 

 ests every working microscopist con- 

 cerns the influence of the shutter 

 upon the penetration of low-power 

 objectives. In the case of all others 

 the penetration to be gained from the 

 objective itself, apart from the power 

 of accommodation of the eye, in any 

 way is, theoretically, so very little, that 

 it is scarcely worth striving for. But 

 with low-powers the effect may be very 

 noticeable. 



An examination of the subject, with 

 Mr. Davis to make the demonstration, 

 has fully satisfied us that the aperture 

 shutter does greatly increase the pene- 

 tration of a low-power objective. We 

 mean by this, an objective of not 

 more than a half-inch focal length. 

 The increase is very strikingly shown 

 in certain opaque objects. Polycys- 

 tina show it very well. The increase 

 in penetration seems due to the shut- 

 ting out of light coming from some of 

 the upper portions of the object, 

 which tends to confuse or over-power 

 the vision of the deeper portions. 



The evidence in support of the in- 

 crease of penetration can be most 

 satisfactorily demonstrated by photog- 

 raphy. Whatever increase is ob- 

 served in a photograph is entirely due 

 to the effect of the shutter upon the 

 objective. We have seen the photo- 

 graphs which Mr. Davis has made to 



