The diaphragm mount should be very substantial. Weak 

 mounts have caused serious discrepancies. E. McCollough reports * 

 a "K." variation in the same instrument, due to various causes, 

 ranging between 101.5 and 102.7. 



The most permanent interval is to be secured by etching lines 

 on a glass diaphragm. In the erecting telescope of the older style 

 with Fraunhofer or Dollond eyepiece this expedient is rather objec- 

 tionable because of the inaccessibility for cleaning; but with the 

 inverting telescope the ocular can be easily removed for this purpose 

 and with the Huyghens ocular, which we use in conjunction 

 with the achromatic triplet erecting system, we preserve the erect 

 image of the field as well as accessibility to the diaphragm. 



The fact that the constant, K, is not always exactly 100 and 

 the fact that damp weather is likely to sag the spider lines and 

 affect the ratio, gave rise to the demand for Adjustable Stadia now 

 met in another way by our cryptic focus and dry telescope interior. 



Fie. 61 



Adjustable Stadia Wires are arranged so that the operator may 



test and adjust his wire interval with the same regularity that other 

 parts of the instrument are rectified; but experience has repeatedly 

 confirmed the opinion that the more readily some adjustments 

 are to be made, the more necessary it becomes to make them. The 

 late Prof. J. B. Johnson, used to say that if fixed stadia some- 

 times change, no argument should be advanced for avoiding the 

 adjustable sort. This fact may also be logically drawn, nevertheless, 



* Enz. Rtc. May 11, /''/.'. 



107 



