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Wenham prism of good quality, and had it properly fitted ; then, finding that 

 the left tube was rather longer than the right, he had the eyepieces 

 differently focussed to suit, having them so marked as to be able to tell the 

 one from the other. Having done this, he found that matters were im- 

 proved, but that there was still something more which required a remedy. 

 To test it he took one of the fine bristles from the maxillary palpi of a blow- 

 fly, but he found that no kind of illumination would make it appear sharp if 

 it were placed on the stage in a vertical position, but if it were placed 

 horizontally across the prism it was perfectly shown. Another experiment 

 was in respect of the stereoscopic effects obtained when the object was in 

 different positions, and the object selected for this purpose was the central 

 pseudo-trachea of the proboscis of the blow-fly. On examining this he 

 found that when it was placed in a vertical position, there was no difference 

 between the stereoscopic effect with and without the prism, except as re- 

 garded the marginal portions of the field, where the eyes were to a certain 

 extent deceived, but when the object was placed horizontally a strongly 

 stereoscopic effect was produced. On the central membrane of the 

 trachea there were a number of small spines, which formed excellent test 

 objects, and if these were placed vertically they appeared foggy, and nothing 

 could be clearly made out about them ; but when seen in the horizontal 

 position their appearance was so changed that it was hardly possible to 

 recognise them as the same objects. In his specimen there was a slight 

 dip or depression in one part of the membrane, which could not be perceived 

 under any illumination with the monocular, but under the binocular, in 

 a horizontal position, it was perfectly well seen, though the same instru- 

 ment failed entirely to show it when the major axis of the lips was in a ver- 

 tical position. He wished to draw the attention of the members of the 

 Club to these matters, in the hope that someone might be able to find out 

 the cause of the difference and to suggest a remedy. 



Professor Charles Stewart said that Mr. Nelson's attention appeared to 

 have been chiefly directed to the effects produced upon lines. He should, 

 therefore, like to ask if he had tested the effects also by points or discs, so 

 as to ascertain if a circle had any tendency to become elliptical in a 

 horizontal direction ? because, though such an effect might be due to some 

 quality of the instrument, it might also be explained by supposing it to be 

 due to astigmatic vision. 



Mr. Nelson said he had not examined points in the same way, but he had 

 experimented with diatoms and found the difference in the stereoscopic 

 effects to be surprisingly marked, especially in the case of Heliope.lta. 



Mr. Karop thought the effect was probably astigmatic, though it possibly 

 might be due to some optical cause, and enquired if Mr. Nelson had made 

 his experiments with more than one prism, as perhaps there might be some 

 defect in this particular prism. 



The Chairman said that, as regarded the difference in the length of focus 

 of the left hand tube, he might mention that Messrs. Powell and Lealand 

 always marked their eyepieces R and L, each being compensated for the 

 length of the tube they were intended for. However perfect. a prism might 



