508 StTMMARY OF CURRENT RESEARCHES RELATING TO 



first, a Wenham, ?. objective, 0'24 N.A., A eye-piece: the second, a 

 Greenough, 25 mm. ol)jective, No. 4 eye-piece : the third a Leitz new 

 binocular, No. 2 objective, 0"17 N.A., and No. 4 eye-piece. There was 

 little to choose between the Greenough and the Wenham, the Wenham 

 being at its best point; the licitz gave a very sharp image, but, 

 strange to say, it appeared much more magnified than the others. (The 

 powers were all re-measured by projection and found to be alike.) In 

 order to bring the apparent size of the Wenham equal to that of the 

 Leitz B eye-piece had to be used. The Wenham image appeared, 

 perhaps, a trifle larger than that of the Greenough, but the difference, 

 if any, was slight. It will be noticed that the aperture of the | on the 

 Wenham was dou1)le that of the 25 mm. objectives on the Greenough, 

 and greater than that of the No. 2 on the Leitz. 



" The powers were then changed, a Baker 1| (O* 2 N.A.) and A eye- 

 piece on the Wenham ; 40 mm. objectives and No. 4 eye-pieces on the 

 Greenough ; a No. 1 (O'll N.A.) and 26 mm. eye-pieces on the Leitz. 

 Here, notwithstanding that the aperture of the 1 1 on the Wenham was 

 almost N.A. 0*2, yet the Greenough gave the better stereoscopic image ; 

 the objective on the Wenham was changed for a low-angle single 

 combination projection objective with the expected result, that there 

 was very little stereoscopic effect left. The Leitz image, as before, was 

 quite sharp. With the Leitz a reduction of the interpupilliary distance 

 of 1| mm. gives full orthostereoscopic effects, and an increase of the 

 distance by 1|^ mm. gives pseudo-stereoscopic effects. 



" There is not the least doubt that the new parallel tube binoculars 

 will give by the Mercer method stereoscopic effects either ortho or 

 pseudo ; and there is the means of easily turning them on or off, or of 

 having no stereoscopic effect, at pleasure. In practice this microscope 

 is most comfortably used with a little — that is, | or f mm. — reduction of 

 interpupilliary distance, which gives a certain amount of stereoscopic 

 effect without flickering of the image or eye-strain. The diameters of 

 the fields were : Wenham 18, Greenough 17, and Leitz 16 hundredths 

 of an inch, and the powers were all the same. The same object was 

 used in all the experiments, viz. an arranged slide of FolycisUnes with 

 the same opaque side mirror illuminator. 



" The higher powers were then replaced, and the diatoms named by 

 'N. E. B.' were examined upon a dark ground with precisely the 

 same results. A Navlcula or Pinnidaria nobilis on the same slide was 

 resolved" in a vertical position, but not when horizontal with the 

 Wenham, as the prism cuts off half the aperture horizontally, but the 

 Leitz with the lens of less aperture resolved it in any position. We 

 see, therefore, that with an object, in which there is fine detail, the 

 Wenham fails, whilst with a coarse object, such as Polycistmes, it is 

 able to hold its own. This account of what these binoculars are capable 

 of doing is quite unbiassed, for I have no axe to grind. Neither the 

 Beck nor the Watson-Ives binoculars are included, as I have never seen 

 them, but the other three are on the table as I write. The measurements 

 of the diameters of the fields are very interesting, and are what no 

 microscopist would have predicted, for the A eye-pieces were those of 

 Powell's No. 1 size, and the others were of the Continental form. 



