1902.] f A Portable Telemeter, or Range-finder. 351 



The binocular is of the prismatic type and is constructed very 

 solidly, and^the inter-objective distance must not vary much when 

 the distance between the oculars is changed to suit different pairs of 

 eyes. A hinged*axis gives the most solid design, and the hinge is so 

 placed as to lie'J&m the plane containing the axes of the objectives. 

 The distance between the objectives is then fairly constant. 



The balloons would become inclined on working the binocular hinge 

 if no special means were used to prevent this. With this object the 

 glass photographs in the focal planes can rotate, and their mountings 

 are provided with slides which keep them parallel. The focal length 

 of the object-glasses is 243 mm. The pitch of the micrometer screw 

 is - 3 mm. The micrometer screw has a drumhead attached, 8 cm. 

 diameter, with the periphery divided into 100 equal parts for purposes 

 of adjustment. On the outer face a spiral groove is cut, making three 

 and a-half turns, and a radially sliding pointer, with a pin running in 

 the spiral groove, enables any range to be read off over three and a-half 

 turns. The scale marked on the groove reads off distances directly, 

 from infinity down to 500 yards. 



(8.) Advantages of Stereoscopic Vision, — A single telescope might have 

 been used, but the difficulty of being sure that the images are single 

 or double is very great and leads to error. With the binocular the 

 two images of the balloon leap together so soon as attention is con- 

 centrated on them, and the balloon seems to be at some distance ; so 

 with the distant object, however faint, the two images leap together 

 when the attention is concentrated on them, and the object's distance 

 can be compared with the apparent distance of the balloon. I find 

 that I can thus appreciate an angle of 6" with the naked eye, or 0"*5 

 with a power of 12. 



(9.) Construction and Mounting of the Prisms. — The prisms are 

 mounted by being sunk and cemented in a steel plate. The clear 

 aperture of the transmitting surfaces is f-inch square. They are 

 mounted in a tube of rectangular section which forms the base. At 

 first I preferred aluminium, but since I was able to obtain steel tubes 

 of the requisite quality and - 02 inch thick I have used them. They 

 are 1 inch high and 1*25 inch broad. The two prisms are ground and 

 polished as one, and afterwards cut ; and I find that with the special 

 glass used the cutting does not distort the surfaces. The whole prism 

 is included in the tube, and this can be done only by making the 

 angle between reflecting surfaces a little less than 45°, this angle being 

 absolutely the same in the two prisms of a pair. Each prism is held 

 to the bottom of the square tube by three screws, which engage in the 

 base plate of the prism, the screw heads abutting on the square tube, 

 and the prism bed-plate being pressed from it by three pieces of clock 

 spring with a hole through which the screw passes. Two of the screws 

 are in line with the base tube, the third is at a right angle. Thus, if 



