32 Transactions of the South African Philosophical Society. 



used for the measurement of celestial photographs, and like such 

 may be of various types. 



In the tyj)e now considered, the plates are set side by side at an 

 inclination corresponding to that of the base line and at heights such 



that corresponding E- 

 points are horizontal. 

 Both plate-carriers can 

 slide about in a hori- 

 zontal direction on a 

 stage formed of a sheet 

 of plate glass g (Fig. 3), 

 which itself can be 

 moved vertically by a 

 double rack and pinion. 

 Any small error in the 

 setting of the plates and in the fitting of the slides will be 

 automatically corrected by the position of the eyes in front of the 

 eyepieces of the viewing microscopes, and by their power of accommo- 

 dation, and does not affect the accuracy of the measurements. 



The measuring microscopes are of low power, and include in their 

 field at least one clear E-square of 1 centimetre side. Their distance 

 apart is adjustable to suit the eyes of the observer. One is fitted 

 with a pair of micrometers at right angles capable of rotation in 

 order to bring the horizontal and vertical wires parallel to the E-lines. 

 The other is similarly fitted with the exception that one horizontal 

 micrometer is sufficient. The runs are adjusted on a scale. 



The centres of the plates are separated to a sufficient distance by 



Fig. 3. 





Fig. 4. 



introducing in each microscope a pair of prisms of total reflection j:> 

 (Fig. 4). 



