Rev. W. Hodgson on New Applications of Prisms, 189 



radius G F is given by the variable portion D F of the screw 

 below the nut D. 



If the table of the instrument (fig. 2) be permanently fixed 

 in a position parallel to the plane of the equator, and if the 

 degree and minute on the graduated circle, correspondent 

 with each face of the prism coinciding with the meridian be 

 observed and registered, the true time may be found within 

 a second or two at any period during the day when the sun 

 is visible, or during the night from observations of known 

 stars of the first, second, or even third magnitude. 



If the instrument be placed with its table horizontal, the 

 observations would give azimuths instead of hour angles. 

 Or, suppressing the graduated circle, and fixing the prism 

 with one of its faces parallel to the meridian, and its axis 

 perpendicular either to the horizon or to the equator, there 

 will be six known directions in which the transits of the 

 heavenly bodies may be accurately determined. 



The rectangular isosceles prism is capable of being em- 

 ployed in this instrument ; and in the course of investigating 

 its advantages, some uses of a prism of that form were sug- 

 gested which appear hitherto to have passed unnoticed. 



In Coddington's Reflexion and Refraction, and possibly in 

 some earlier optical works, it is stated that a feeble but very 

 distinct second internal reflexion may take place in a trian- 

 gular prism, when one angle is a right angle, and the other 

 two each half a right angle. It is not, however, stated that 

 the feebleness of this image may be remedied by silvering 

 the base of the prism. If this plan be adopted, the rectan- 

 gular prism becomes at once a camera lucida. Fig. 4 shews 

 the course of the ray in this case, and it is obvious from 

 fig. 5 that there is another, though for some purposes less 

 convenient direction in which the prism will serve the pur- 

 pose of the camera lucida. There is also this advantage in 

 the rectangular prism, that if it is desired to take a reversed 

 copy of the object, this is given by the single internal re- 

 flexion, and either may be taken at pleasure. 



In this, and in Dr Wollaston's camera, the incident and 

 the emergent rays are accurately perpendicular to each other, 

 and it is somewhat strange that a little instrument termed 



