SUGGESTED IMPROVEMENT IN BINOCULAR MICROSCOPES. 38T 



Once having grasped the great feature that parallactic 

 displacements in the image of successive layers of the object 

 are the chief means by which stereoscopic effect is produced 

 when the ordinary form of binocular microscope is used, some 

 interesting deductions follow. It is. in the first place, clear that 

 we can only get good results if these displacements tend to 

 change the image pictures in the same way as the image pictures 

 of an object change when viewed from the different station points 

 of right and left eye respectively. Xow if we take two objects, 

 matches, for example, and hold them behind one another, looking 

 at them with right and left eye alternately, if we consider the 

 position of the nearer match in relation to the one further away 

 it appears shifted further to the left when viewed with the right 

 eye, or more to the right when viewed with the left eye. If, 

 however, we consider the position of the further match in re- 

 lation to that of the nearer one, the right eye sees it shifted 

 toward the right, the left eye towards the left. But we have 

 seen from our diagram (Fig. 1) that any portion of the right half 

 of the microscope objective just shifts the images the other way 

 — viz. the image of the nearer planes to the right, and those 

 further awav to the left. It is for this reason that in instru- 

 ments like the Wenham binocular the pencils of rays from the 

 two halves of the objectives are made to cross over so that the 

 left- eye picture is formed by the right side of the objective, and 

 vice vers&r If, however, erecting eye-pieces or an "erecting prism 

 is used, as in the case of the Stephenson binoculars, then, since 

 these already change all the pencils or rays from the right to 

 the left and vice versa, the necessity for making the rays cross 

 over no longer exists, and the image from the right half of the 

 objective is brought to the right eye, that from the left half to- 

 the left eye. 



The recognition that parallactic displacements in the correct 

 direction are the primary and essential factors for getting 

 truthful stereoscopic effect when one objective has to supply 



* Mention should here be made of a fact which I ara indebted to Mr. 

 Conrad Beck for pointing out. since this paper was read— viz. that the 

 prism in Wenham binoculars is usually worked to such an angle that the 

 object seen exactly in the centre of the field with the right eye, is not 

 exactly in the centre of the field seen by the left eye. This naturally has 

 a bearing on the stereoscopic effect, the general result, however, appearing 

 to be that due to a displacement of the whole images bodily. 



