On a Stereoscopic MetJiod of Pliotograpliic Surreijing. 33 



The micrometers might also be used in the position of the plates, 

 ■giving more room for the screws and greater facility in the reading 

 ■of their heads, and the plates themselves set further back, behind an 

 additional lens, as in the Cambridge measuring machine recently 

 described by Mr. Hincks {Monthly Notices, Ixi. p. 444). 



The zero wires form a frame fitting an E-square, as in Sir David 

 Gill's machine used at the Cape Observatory {Montldy Notices, lix. 

 p. 61). 



For convenience, the whole arrangement is tilted at an angle of 

 45^, and the light illuminating the plates reflected by mirrors vi from 

 :a window at the back of the observer. 



The setting of the plates may be effected by turning a micrometer 

 to the inclination of the base by means of a graduated circle, and 

 making both sets of R-lines agree in inclination and height with the 

 micrometer wires. The second micrometer is then set by making 

 its wires parallel to the vertical B-lines on either plate. 



The vertical E-lines are combined by the microscopes, but the 

 horizontal lines only wdien the distance between the centres of the 

 pictures is equal to that between the microscope object-glasses.''' In 

 making a measurement the plates are moved by the slow motion 

 screws on the slides of their carriers and of the stage until the zero 

 square of one microscope fits an E-square of the corresponding 

 plate, and the zero wires of the other microscope coincide with a 

 pan' of vertical E-lines on the second plate. The points in the 

 held of view may then be bisected without disturbing the zero 

 settings. 



The co-ordinates of any point on the plates are given by the direct 

 readings of the micrometer heads added to the value of the E-lines 

 considered. The stereoscopic difference results from the difference 

 of the x's on the two plates. 



Bange of the Method. — In practice, the range of the method would 

 be limited by the blurring of distant detail by light diffused in the 

 atmosphere. This " aerial perspective " is reduced by the use of 

 orthochromatic plates and an orange screen cutting off the rays of 

 shorter wave-length which form the blue haze, but even then the 

 effective range w^ould probably not exceed some 5 miles or 8 kilo- 

 metres. 



On the other hand, the difference in phase of the objects would 

 prevent their ready combination at distances less than three to four 

 times the length of the base. The view would then correspond to 

 that of a model seen with the eyes at a distance of 10 inches from 

 the nearer edge. 



'O" 



[Should be " between the lower optical axes of the microscopes."] 



3 



