1072 SUMMABY OF CUREENT RESEAECHES RELATING TO 



witli a sharp focus upon the view of the shell which I desired, I have 

 taken transparencies from them by contact, and using these last as 

 negatives from which to print the paper prints, I have found that 

 these last are, according to my notes, what the former should have 

 been if there were no difference between the visual and the actinic 

 focus. A few of these have been prepared for exhibition to the 

 Society. The prints taken from the second plates are marked ' posi- 

 tives ' of the originals, and are in fact the true representation of the 

 object as I saw it when taking the original photograph. They are — 



No. 66. Navicula serians Kiitz., taken with a Spencer 1/16 in. 

 balsam angle 125°, with No. 118 as the positive from it. 



No. 60. Pleurosigma formosum W. Sm., taken with a Spencer 

 1/10 in. balsam angle 108°, with No. 122 as the positive from it. 



No. 83. Pleurosigma formosum W. Sm., taken with a Wales 

 1/15 in. balsam angle 82°, with No. 119 as the positive from it. 



No. 110. Pleurosigma halticum W. Sm., taken with a Zeiss 

 1/18 in. balsam angle 116°, with No. 113 as the positive from it. 



The objectives are all of the first class, and it is safe to assume 

 that what holds true with them will be found true with any of our 

 best glasses. In taking the original photographs, I used a plain 

 plate of glass instead of the usual ground-glass screen in the camera, 

 and focused by the aid of a Dorlot focusing glass. 



The examples to which I have referred would seem to warrant 

 the conclusion that in using high-power objectives, the difference 

 between the visual and the actinic focus is the equivalent of that 

 between a positive and negative image of the object, when the details 

 have passed a certain limit in fineness. But some experiments, made 

 for the purpose of finding how far the tube of the Microscope must 

 be moved to secure the proper actinic focus upon the sensitive plate, 

 have had such unsatisfactory results as make me unwilling to venture 

 any positive conclusion, but content myself with stating the facts 

 above given, until further investigations which I am making shall be 

 completed. 



In the course of the experiments referred to, I noticed that the 

 image taken on the plate was apparently of a lower plane in the 

 object, than the visual one which I was seeking to get. This was 

 shown in the diatoms with a convex surface, by the sharper image, in 

 the print or plate, of areolae nearer the margin of the object than 

 those upon which I had focused. It showed also that the difference 

 seemed to be the same in kind as in the use of low-power objectives, 

 with which it is necessary to raise (withdraw) the tube after getting 

 a sharp visual image of the object. Acting upon this, I tried in 

 several instances the gradual raising of the tube, taking pictures at 

 slightly varying departures from the visual focus, until the image 

 was quite spoiled and blurred to the eye. I made some series of as 

 many as five or six plates thus progressively varying, but without 

 satisfactorily establishing any point (different from the visual focus) 

 at which the objective should be placed to secure in the photographic 

 image the true characters of the visual one. I was surprised to find 

 at what a distance from the visual focus a sharp image could be 



