Construction of Wollastons Goniometer. 219 



provided with a silvered mirror attachable to the axis, the in- 

 strument would become an efficient reflecting circle. 



If all the observations be made on one object, that part of the 

 parallax which depends on the distance of the axis from the 

 point at which the reflection takes place on the permanent re- 

 flector will be constant, and will influence all the readings in the 

 same way, it may therefore be entirely neglected ; only that, 

 when the object is very near, care must be taken to use the same 

 part of the surface, or to direct the eye in a fixed way in regard 

 to the parts of the goniometer : this, however, only when the 

 distance is a few feet. The parallax, depending on the distance 

 from the axis of the instrument to the face of the crystal, must, 

 however, be attended to. In the direct observation this parallax 

 will affect the reading in one way ; in the back observation the 

 same parallax has the opposite effect, so the amount of the two 

 readings is freed from its influence; and thus half the difference 

 between the sum of the readings in the one face and the sum of 

 the readings in the other face of the crystal will be their incli- 

 nation. By using this process, the error of centering is entirely 

 corrected ; it is, however, troublesome when the crystal is mi- 

 nute, and the repetition of the measurement by the direct me- 

 thod is preferable, the object being taken at some considerable 

 distance. 



The surface of the permanent reflector must not be too bright, 

 otherwise the light from it will be too strong, and prevent the 

 distinct vision of the other image. By moving the eye a little 

 backwards and forwards a position can be found which will give 

 to both images the like brightness, and thus facilitate the esti- 

 mation of the coincidences. 



In conclusion, I remark, that the stops affixed to the instru- 

 ment, as ordinarily made, impede very much the work ; on 

 which account I would recommend their removal from it even 

 in its usual state. 



