1885.] 



MICROSCOPICAL JOURNAL. 



169 



then make the image sharp on the 

 distiint screen without the eye-piece 

 by means of a supplementary lens. 

 Mr. Zeiss attempted to do this by- 

 providing suitable correcting lenses 

 to be screwed into the back of the 

 objective, which were calculated to 

 correct the aberrations when the 

 screen is at stated distances away. 

 A better plan, however, was em- 

 ployed by Dr. Woodward, who fully 

 understood the problem and solved 

 it in a satisfactory manner. He made 

 use of an amplifier by Tolles, which 

 he found to be most satisfactory of all 

 at the time.* The amplifier was 

 placed in a draw-tube, so that it could 

 be moved out or in as required. He 

 found that for any given position of 

 the screen there was a corresponding 

 position for the amplifier at which 

 the image was as sharp and perfect 

 as when observed with an occular, the 

 objective meanvvhile remaining un- 

 moved. Herein is the secret of Dr. 

 Woodward's unexcelled • work in 

 photographing the most difficult test- 

 objects. He used no small plates, 

 but the images of A. pellucida were 

 ten inches in length, clear and sharp 

 throughout. It is safe to assert that 

 such pictures cannot be taken with 

 an ocular, or even without the cor- 

 recting lens properly applied. These 

 facts are either not generally known, 

 or they are sadly neglected by those 

 who have most need to apply them ; 

 for in these days of photographing the 

 most difficult of all objects, thev ari- 

 ous forms of bacteria, the utmost sharp- 

 ness of definition is required if the re- 

 sults are to possess permanent value. 

 We now come to another question 

 of importance in connection with this 

 subject, viz., how much shall we 

 magnify with each objective.-* We 

 already know that there is a limit, 

 clearly determined by calculation for 

 each objective, dependent upon its 

 numerical aperture, beyond which 

 no further amplification will bring 

 out additional details. Beyond that 

 limit increase of magnification only 



•See this journal, vol. i, p. 5. 



enlarges the details then visible. Ex- 

 perience clearly shows, also, that as 

 we increase amplification beyond a 

 certain point, with every objective, 

 we lose in sharpness of definition. 

 This should be borne in mind in 

 photographing with the microscope. 

 The negative shovdd be taken with 

 only such magnification as gives per- 

 fectly sharp definition. By far the 

 greater part of the general photo- 

 graphic work of microscopists is 

 done with powers not greater than 

 250 diameters, and rarely indeed is 

 400-=;oo diam'eters required. 



This, however, will depend upon 

 the work to be done. In photograph- 

 ing diatoms, Mr. J. D. Cox has used 

 powers of a thousand and twelve hun- 

 dred diameters very successfully, and 

 Dr. Woodward's celebrated photo- 

 graphs of Ampkipleura pellucida 

 were taken with magnifications of 

 more than two thousand diameters. 

 The magnification of certain photo- 

 graphs in our possession is 2,700 to 

 2,900 diameters with a ^-inch objec- 

 tive, and 3,400 with a ^'^-inch. The 

 frustules measure about ten inches in 

 length. Considering this fact and the 

 size of the pictures, embracing as they 

 do the entire length of the frustules in 

 perfect focus throughout, we regard 

 these photographs as the most perfect 

 that have yet been produced. They 

 conclusively demonstrate that it is 

 practicable to make photographs with 

 the highest powers which shall equal 

 the best definition the lens is capable 

 of giving with such magnifications ; 

 but as there is a limit to the sharpest 

 definition of every lens as ordinarily 

 used in observing with an ocular, 

 there must also be such a limit in 

 photography. The rational method 

 would seem to be to get a sharp nega- 

 tive with as much magnification as 

 possible, and make enlargements from 

 that if required. In working with 

 an amplifier enlargements will seldom 

 be desired : but without the amplifier, 

 either with or without an ocular, the 

 limit of successful direct magnifica- 

 tion is much reduced. 



