Royal Microscopical Society. 167 



IV. — Stephenson a Erecting Binocular. 



By J. W. Stephenson, F.E.A.S., Treasurer E.M.S., and Actuary 

 to the Equitable Assurance Society. 



(^Read before the Eotal Microscopical Society, March 6, 1872.) 



Plate XV, (Lower part). 



In concluding the description of an erecting binocular microscope, 

 which I had the honour of bringing before the Society in the year 

 1870, I remarked that possibly improvements in the arrangement 

 or angles of the prisms might subsequently be devised. 



It soon became evident that this was so, and that the mass of 

 glass used in the lower prisms, by which the light on leaving the 

 objective is divided, and the image laterally inverted, might be 

 considerably diminished by bringing the latter closer to the back 

 combination of the object-glass, and also that by this means higher 

 powers than those which I had hitherto employed might be used ; 

 at the same time, for reasons to be hereafter explained, it appeared 

 desirable to alter the angles at which the bodies are inclined to 

 the perpendicular, from 75° to 66|°. 



Under the present arrangement, two truncated rectangular 

 prisms are still employed for dividing the light and inverting the 

 image, but their dimensions are so far reduced that they are capable 

 of being inserted into the object-glass itself (Plate XV., lower part) ; 

 this is accomplished by placing them in a small brass tube, which 

 is fixed in and projects beyond the nozzle of the instrument, but 

 without in any way afi'ecting the screw ; they are each • 68 of an 

 inch in length, "412 of an inch in width, and '2 of an inch in 

 thickness, and are inclined to each other at an angle of 4|° ; this 

 makes the angle between the bodies 9^°, and the imaginary point 

 towards which the eyes converge nearly 15 inches. 



This angle was selected as being generally the most suitable, 

 bearing in mind the desirability of maintaining the ordinary length 

 of body, and of giving an amount of convergence which should 

 relieve the eyes of the strain not unfrequently experienced in the 

 use of a binocular. In some instances where the eyes of observers 

 are either closer together or wider apart than usual, it may be 

 desirable to deviate from this angle in either direction, to meet 

 the requirements of a particular case. 



It is a matter of vital importance that the prisms be not too 



EXPLANATION OF PLATE XV. (Lower part). 



The Plate represents an ideal section of the front of the microscope, removing 

 half of the binocular prisms, the objective, and their fittings. The objective is 

 shaded with oblique lines. 



