The Abbe Erecting Prism was designed by Dr. Ernest 

 Abbe of the Zeiss works some 20 years ago for the purpose of pre- 

 serving the original relative proportions of the field without chang- 

 ing the direction of the original beam.* As shown in Fig. 118, it 

 is a solid glass prism of four reflecting surfaces which not only 

 propagate a reflected beam in its original direction but erect the 

 image produced by the objective, while in the roof is preserved the 

 relative lateral positions. Abbe used it as a substitute for the Porro 



Fig. 118 



system of prisms now used in hand binoculars, and it has been used 

 in transit telescopes between the objective and diaphragm as a sub- 

 stitute for the ordinary erecting lens system. Its advantage here 

 lies in the fact that it adds an appreciable increment to the focal length 

 of the objective, contributing also somewhat to other optical 

 qualities in consequence. On the other hand, it absorbs somewhat 

 more light, and studying the effects of remotely possible displace- 

 ment, its important relationship to the collimation adjustment is 

 apparent. Where the higher magnification seems indispensible 

 with small instruments w r e have used it successfully, but the image 

 is not quite so crisp and the precaution of collimation tests in 

 marked changes of temperature should be observed. 



The Penta Prism is unquestionably the best suited for use 

 in connection with telescope objectives. It was designed by Col. 

 Goulier in 1864 but was really not an invention, for it utilized the 

 basic principle of the "optical square" or the principle of the 

 relationship between the index and horizon mirrors of the sextant. It 

 is a well known fact that when a ray is reflected twice in the same 

 plane, the second reflection makes an angle with the incident ray 

 equal to twice the angle included between the reflecting surfaces. 

 Fig. 119 shows two ray paths being reflected successively between 

 the silvered surfaces with an idea of emphasizing the fact that what- 

 ever the original direction of the incident beam, the projected ray 

 will pass out of the penta prism at right angles to that direction if 



reinsblatt dtr dfittschfn Geselhchaft fur Met),, u. Optik, IM 

 eder Mi*lerne Optischen lnstru>n,nt, , A. (,'leic/irn, 1911, p. 152. 



I'M 



