The Variable Power Eyepiece, supplied solely by this house, 

 is an accessory whose purpose is to add flexibility to the tele- 

 scope. The ordinary surveyor's telescope 

 is provided with a single eyepiece which gives 

 a degree of magnification which must be a 

 compromise between that value which enables 

 the longest sight to be taken and the value which 

 affords the brightest image. Even if this com- 

 promising value be intelligently chosen, the 

 engineer is losing some of the possibilities of 



his instrument. On bright days he could use still higher power and 

 take longer sights while on dark days and by mid-afternoon in 

 winter he is seriously hampered by the dullness of the image, if not 

 obliged to give up field work. 



By constructing the variable power eyepiece, we have given 

 the engineer the power to choose for himself the degree of magnifi- 

 cation best adapted to the light conditions of the moment. In the 

 discussion of the brightness of image, w r e found that the illumina- 

 tion was inversely proportional to the square of the magnification. 

 Our variable power eyepieces have a range of magnification such 

 that the ratio of the lowest to the highest power is in some cases 1 : 

 2 and in other 3:5. In the first case, then, we give the engineer 

 the power to vary the illumination in the ratio of 4 : 1 while in 

 the second, the range is somewhat less than 3 : 1. This means a 

 substantial increase in the length of the engineer's working day in 

 the field. To the gain in this respect must be added another gain 

 made possible by the use of the low power. When the air is boil- 

 ing under a hot sun the ranging rod often appears so unsteady that 

 readings are impossible. The use of the lowest power will 

 materially reduce the trouble and enable work to be done which 

 without the variable power eyepiece would have to be postponed. 



The principle involved in the change of magnification is 

 identical with that employed in the old pancratic eyepiece. The 

 eyepiece as a whole is of the erecting type and consists of an ocular 

 and an erecting system, and ahead of these, mounted absolutely in- 

 dependent of the eyepiece, is the cross wire reticule. By altering the 

 distance between erecting system and cross wires and then moving 

 the ocular alone until the cross wires are again in focus, the focal 

 length of the eyepiece, and therefore the power of the telescope, has 

 been changed. The cross wires in no way participate in this move- 

 ment but remain in fixed position attached to the telescope tube 

 and collimation is no more affected by this operation than by 

 leveling the instrument. 



To operate the attachment, first set the knurled ring to the 

 magnification desired and then focus the ocular so that the cross 

 wires are brought into focus. The power may be changed, while 



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