recorded vertical angle setting of FF on the scale B. For vertical 

 angles greater than 20, scale D is used with a special scale for rod 

 intercepts E. 



"The inventor claims an accuracy in plotting horizontal dis- 

 tances equal to the case where the distance is first computed by slide 

 rule and transferred to the map by plotting; and while this would 

 answer for ordinary stadia side shots, it would not seem to eliminate 

 the necessity of computing the traverse lines for closure. The 

 accuracy of elevation is claimed to be \\\ ft. The Stadiagraph will 

 plot all vertical angles up to 30 and all elevations up to 130 ft. 

 The plotting length for horizontal distances is 10 in." 



Prismatic Stadia Telescope 



Prof. Richards pointed out * that one of the sources of error in 

 stadia observations was due to the fact that in focusing the ordinary 

 telescope, between long and short sights, the ratio between the 

 wire interval and the various conjugate foci produced inconsistencies 

 in the value of the angle subtended by the wires. 



The negligible error in the value of c may be overcome by 

 ocular focus but the proposal that /or K vary at all in the process 

 of objective focusing is open to question. 



He proposed, however, to dispense with the diaphragm interval 

 and substitute an objective prism ground to an angle sufficient to 

 deflect the sight line in the proportion of 1:100 at all distances. If 

 a prism of this description is mounted just in front of the objective 

 so as to cover half its area, there will be received at the eyepiece a 

 double refracted image favorable to the plan of bringing two targets 

 into apparent coincidence. The targets will not form a contact, 

 as effected by the prismatic refraction, until the distance between 

 them is proportional to the distance from the instrument. 



The scheme was patented in France under the name of Diastimo- 

 metre Sanguet. The prism was hinged to an objective mount so that 

 it could be thrown into or out of the sight line at will. 



The Anallatic Telescope 



The optical arrangement by which all observations are auto- 

 matically corrected to the center of the instrument was the ingenious 

 invention of Prof. Ignaz Porro, of Milan, who put it into practical 

 use in 1823 t 



In this type of telescope, the focus of the objective, O, is a 

 constant quantity. Consequently the various rod intervals sub- 

 tended by the constant angle aFb, are directly proportional to 

 the distance, and the (c -f f) constant of the usual system is thereby 

 discarded. 



* R. H. Richards, M. f. T., in Trans. A. f. M. E., 1X91. 

 f La Tacheomtirtt, on rart dt Levtr Its Plans el dt l-aire Its Nivellement 

 Turin, 1850. 



IIS 



