1885.] 



MICROSCOPICAL JOURNAL. 



195 



ness. But some experiments, made 

 for the purpose of finding how far the 

 tube of the microscope must be 

 moved to seciu'e the proper actinic 

 focus upon the sensitive phite, liave 

 had such unsatisfactory results as 

 make me unwilling to venture any 

 positive conclusion, but content my- 

 self 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 areolse 

 nearer the margin of the object than 

 those upon which I had focussed. It 

 showed also that the difference seem- 

 ed to be the same in kind as in the 

 use of low power objectives, with 

 which it is necessary to raise (with- 

 draw) the tube after getting a sharp 

 visual imagb 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 progressive- 

 ly varying, but without satisfactorily 

 estiiblishing any point (different from 

 the visual focus) at which the object- 

 ive should be placed to secure in the 

 photographic image the true char- 

 acters of the visual one. I was sur- 

 prised to find at what a distance from 

 the visual focus a sharp image could 

 be taken, but it was not the image for 

 which I was in search. Examples 

 of this sort are among the prints 

 which I will exhibit to the Society. 



I design to add to my experiments 

 on the subject, the examination of the 

 effect of changing the focus of the 

 focussing glass to correspond with 

 the difference between the visual 

 image of a diatom showing light dots 

 or areolae and that which shows dark 

 ones. Everj'body has noticed that a 



slight change of focus with a high 

 power produces this change of ap- 

 pearance, and if the focussing glass 

 were adjusted for the image which is 

 complementary to the one desired 

 and then the focussing done in the 

 usual way, the result might be that 

 which is sought. It has at least 

 seemed worth the experiment, but a 

 press of other work has prevented my 

 making a satisfactory test of it before 

 the time of our meeting. 



American Society of Microscopisis. 



On the second day the proceedings 

 were opened by some remarks by Dr. 

 Detmers on the poisonous dried beef 

 which caused violent sickness in 

 Momence, 111., some time ago. Dr. 

 Detmers found abundance of bacteria 

 in the specimen he examined. 



Dr. Thomas Taylor demonstrated 

 his methods of distinguishing fats 

 with the microscope. This subject 

 attracted considerable attention and 

 a committee was appointed -to ex- 

 amine the processes and report upon 

 the results. The committee was first 

 composed of Dr. Fell, Dr. Detmers 

 and Mr. Vorce, but Dr. Curtis and 

 Mr. Atwood were afterwards added. 

 The work done by the committee be- 

 fore separating indicates that Dr. 

 Taylor's methods are reliable. The 

 members of the committee are indi- 

 vidually at work on the subject. The 

 method of observation was described 

 in these columns last month. 



Mr. J. Kruttschnitt presented a 

 paper entitled ' Pollen Tubes Again.' 

 This was followed by a contribution 

 by Dr. Lester Curtis describing the 

 method of cultivating micro-organ- • 

 isms. Dr. Curtis' method is to take 

 a good potato, clean and boil it, be- 

 ing careful to get a potato that is not 

 mealy. The potato is then immersed 

 in a solution of corrosive sublimate. 

 A scalpel is heated to a red heat and 

 laid edge upward, as the bacteria 

 which continually float in the air are 

 less liable to attach themselves to the 

 sharp edge than to the side or back 



