E. M. NELSON ON BINOCULAR MICROSCOPES. 379 



when bacilli in tissue are examined with the Leitz binocular, a 

 ^ and the Mercer method, a beautiful picture of them in 

 perspective projection will be seen as well as of the cell nuclei 

 which appear spherical as marbles. 



It is a good plan when working with this new binocular to 

 turn on the Mercer effect and when the form of the image has 

 been mentally grasped to turn it either wholly or partly off, 

 for when the stereoscopic form of an object has once been 

 realised by the mind re can be retained, although the optical 

 conditions which gave rise to it have been removed. Some will 

 have noticed, when looking at parquetry representing cubes, that 

 if the effect when first noticed is intaglio it is a matter of some 

 difficulty to reverse this mental image so that the cubes shall 

 appear to be in alto-rilievo. 



I asked Messrs. Leitz to make me a couple of tubes to slide 

 over their tubes, by which means tube-length adjustment can 

 be accomplished. The tubes can be drawn up and down over 

 the fixed tubes and the eye-pieces also can be partially drawn 

 out, as the tubes are sprung both top and bottom. Without 

 these tubes it was not possible to obtain a critical image with 

 Messrs. Leitz' own objectives for the Continental short tube. 



The great charm in these new binoculars consists in the 

 sharpness of the image combined with ease and comfort of vision, 

 hence the need for lens correction either by alteration of tube 

 or by screw collar. The sharpness of image in my instrument 

 at least is very little behind that of a monocular, for it requires 

 a delicate test to perceive any difference at all, and often a pair 

 of 18 compensating eye-pieces have been used with advantage. 



With a ^ inch objective and upwards, these new binoculars 

 have the field all to themselves, as no other binocular for sharp- 

 ness and crispness of image can for a moment compete with 

 them. With low powers and 1^ inch eye-pieces and a slight 

 Mercer effect they give lovely images, but, as was hinted above, 

 with the Mercer effect one must alwaj^s be on one's guard against 

 hyper-stereoscopism. Recently a shock was experienced on finding 

 that a Radiolarian which appeared under the Mercer effect as 

 round as an orange, when viewed on edge was shaped rather like 

 a mince pie. Here the Wenham gave the truer image. 



Latterry, even the Greenough, which is known to give beautiful 

 images, has been suspected of hyper-stereoscopic tendencies. 



Journ. Q. M. C, Series II. No. 75. 27 



