ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1075 



fitted ; tlien, finding that the left tube was rather longer than the right, 

 he had the eye-pieces differently focused 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 improved, 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 regard to 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 pro- 

 boscis 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 regarded the 

 marginal portions of the field whei'e the eyes were to a certain extent 

 deceived, but when the object was placed horizontally a strongly stereo- 

 scopic 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 recognize 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 instrument failed entirely 

 to show it when the major axis of the lips was in a vertical position. 

 He also tried dintoms, and found the difference in the stereoscopic 

 effects surprisingly marked, especially in the case of Heliopelta. 



In a later communication * Mr. Nelson deals more fully with the 

 case of the proboscis of the blow-fly as follows : — 



" I wish tliat every possessor of a binocular would try the 

 following experiment. 



Place the proboscis of the blow-fly, squeezed flat in balsam, in a 

 vertical position, and examine it binocularly with, say, a 2/3 in, 

 objective, and let the attention be concentrated solely on the two 

 main vertical cut suctorial pipes. Now let the observer carefully 

 examine those Avith a view to determine the amount of stereoscopic 

 effect the binocular gives to them. Let me warn him against letting 

 his eye cheat him by giving those suctorial pipes a stereoscopic effect 

 which they do not possess, derived by contrast with other parts of the 

 field. He must, to make this experiment correctly, resolutely shut 

 his eyes to everything else in the field except those suctorial pipes. 

 I feel sure that no candid observer correctly performing this exjieri- 

 ment will be able to detect any more stereoscopic effect on that object 

 than if it were examined monocularly. Of course, there will bo 

 Btercoscopic effect to a certain degree, as there will bo also in the 



♦ Eng. Mccb., xlii. (1885) pp. 202-3. 



