Meisch, on Xachet's Stereo-Pseudoscop'ic Microscope, ll'-j 



other, to produce either stereoscopic or pseudoscopic effects, 

 and on account of its small size can be withdrawn from the 

 object-glass altogether into a small recess, and thus convert 

 the instrument into a uniocular microscope, thus removing 

 the first two objections. The reduction in the size of the 

 first prism enables the second prism g, i, h to be brought 

 close into the object-glass, and thus the second body can be 

 placed at such an angle to the first that the draw^-tubes can 

 be used, as in Wenham^s instrument, to regulate the distance 

 of the eyepieces. The seconct prism and body may thus be 

 made fixtures, and not only the danger of getting loose by 

 work be done away with, but if the eyepiece be once pro- 

 perly adjusted for focus, they afterwards move simul- 

 taneously, and can be focussed together as in an ordinary 

 instrument, thus removing the third objection. 



It may be asked, wdiat advantages does this form of in- 

 strument possess over that in ordinary use ? I was at first 

 inclined to think, that beyond being a pretty illustration of 

 the manner in which the eyes may be deceived by ])re- 

 senting to them the wrong side of an object, not any. But 

 closer acquaintance with the instrument has convinced me, 

 not only that this is of practical value, but that there are 

 other advantages besides. When there is a very shght 

 difference in the planes in which two objects or parts of 

 an object lie, it is difficult, even with the binocular instrument, 

 to say if two parts, a and b, are exactly in the same plane. 

 One thinks a may be above b, but does not feel sure ; if, how- 

 ever, on moving the prism from one side of the object-glass 

 to the other, a distinct difference is observable, the doubt is 

 converted into a certainty. Another advantage is that, owing 

 to the prism ^, i, h having an independent adjustment, it is 

 easier to get a perfect coincidence in the position of the two 

 images, together with a perfect reflected image, than where 

 no independent adjustment is possible after the prism is once 

 ground. This perfect coincidence of position is of compara- 

 tively little importance to those who have strong muscles to the 

 eye, but to those who, like myself, have a weak internal 

 rectus muscle, it makes all the difference between comfort 

 and discomfort. 



I may mention that I have met with several persons who 

 have great difficulty in using the ordinary binocular micro- 

 scope, who use the instrument now brought before the Society 

 with ease and comfort. 



