348 



one got a better representation of the object with the long-tube 

 form. Some time ago, wanting to obtain the greatest possible 

 am, Mint of magnification, he used an old-fashioned long-tube 

 microscope, which was fitted with an erector, and thereby ob- 

 tained the equivalent of a tube-length greater than any supplied 

 by modern opticians. He was able to obtain a magnification 

 of x 150. using only a two-thirds. He, of course, agreed that 

 the short-tube form was best for bacteriological and similar work. 



Mr. ( lonrady considered there was little to choose between the 

 two forms, beyond the advantage mentioned by the President 

 regarding the use of the short form in vertical observation. He 

 recollected one advantage for the long tube claimed by Nelson, to 

 tlif effect that an observer using a 10-in. tube and keeping both 

 eyes open, as he should do, would be able to keep one eye at the 

 microscope, and could watch an object on the stage with the 

 other. The angle of the larger eye-piece of the long tube, having 

 regard to increased amplification, would be about the same as 

 with the short form. The practical diameter of the body tube 

 depended on the corrections of the objective. 



Mr. Rheinberg said the image obtained with the short tube 

 was considerably sharper than with the long. All images might 

 be considered as a mosaic of little discs, the size of the discs 

 increasing with the magnification, and also with the tube-length, 

 the smaller and sharper picture being composed of small discs, 

 while with the 10-in. tube the picture was formed of larger discs. 

 It was possible that it might sometimes be advisable to employ 

 a large-disc image. The size of the little discs varies with the 

 aperture of the objective — not the numerical aperture, but the 

 effective back aperture — this being the reason why a 1-in. will 

 give a much sharper image and will stand higher eye-pieceing 

 than, say, an g-in. Therefore, discretion was advisable as to the 

 tube-length to be employed. He agreed with the President that 

 both forms had their advantages. 



The meeting then concluded with the customary votes of 

 thanks to the President, the Auditors and Scrutineers, and to the 

 I iommittee and Officers for their much-appreciated services during 

 the past year. 



Jowrn. Quekett Microscopical Club, Ser. 2, Vol. IX., No. 58, April 1905. 



