6 Transactions of the Society. 



it would also include normal binocular ultra-microscopy. This 

 problem has been solved, and it should be observed, that a notice- 

 able deterioration of the image, which was to be feared owing to 

 the large bodies of glass necessary, has not taken place 



Fig. 1 shows the external appearance of the instrument. The 

 tube has become a flat box containing the system of prisms. At 

 the upper end are situated the two eye-pieces, whose distance apart 

 can be regulated to suit the eyes of the observer by means of a 

 milled head which actuates two toggle levers inside the box. The 

 distance apart can be varied between 54 and 74 mm. The eye- 

 pieces slide in guides so constructed that no dust can enter the 

 prism-box owing to this movement. On the left-hand side is a 

 millimetre scale, permitting the correct setting to be made for 

 observation. 



As the two eyes are generally of unequal strength, it was found 

 necessary to fit an independent adjustment on one of the eye-pieces. 

 This can be placed in the right- or left-hand eye-piece. The usual 

 way is to focus by coarse and slow adjustment, using the fixed 

 eye-piece only, then the proper setting apart is given to the two 

 eye-pieces, and finally, if necessary, a further adjustment by the 

 movable eye-piece is made. All kinds of eye-pieces may be used. 

 The eye-piece corresponding to the eye which is more shortsighted 

 is of course set a little lower than the other. 



The simple internal arrangement is shown in fig. 2. In the 

 cemented prism nearest to the objective will be found a semi- 

 transparent coating of silver, which effects the above-mentioned 

 physical division of the pencils of rays. There is nothing novel 

 about the arrangement of the prisms ; on the contrary, it has been 

 variously applied to optical apparatus in this and other modifica- 

 tions. It is derived from the so-called " Swan cube." The semi- 

 transparent film of silver also finds application in physical 

 instruments. It is technically possible to adjust the film of silver 

 so exactly that the transmitted and reflected light are practically 

 of equal intensity. The thickness of the glasses is chosen so that 

 the length of the optical paths are equal both to the right and to 

 the left, thus securing equal magnification. 



The new Microscope has another peculiarity, namely, that the 

 two eye-pieces are parallel. We know that in the human eye the 

 actions of accommodation and adjustment for convergence are 

 coupled so as to work together. A convergent action generally 

 calls for an effort of accommodation corresponding to an approach 

 of the object under observation nearer to the eyes, and vice versa. 

 If, therefore, the eyes are forced to convergent action a certain 

 accommodating effort is forced upon them, and this one would 

 prefer to avoid because the eye-pieces of Microscopes are designed 

 for the emergence of parallel rays, that is for an unstrained eye. 

 Such observational work is very tiring when continued for any 



