ZOOLOaY AND BOTANY, MICKOSCOPY, ETC. 431 



to a still greater inconvenience. Iceland spar is very soft, and the 

 optician has much difficulty in producing true surfaces with it. The 

 polishing always deforms the surfaces notwithstanding all the care 

 and skill of the workman ; and the slight deviations which cannot be 

 avoided in the surfaces affect the direction of the transmitted rays the 

 more injuriously, according as the angles of incidence are large. 



In fact, whenever the rays form after their passage through the 

 prism a real or virtual image, this image is always confused and 

 badly defined. But it is especially when the image has to be again 

 amplified that the defects in the surfaces give rise to the most trouble- 

 some consequences. 



These considerations led the authors to think that the first thing 

 to be done by way of remedy was to give to the faces of incidence and 

 emergence, a direction normal to the axis of the prism. This direction 

 allows the rays which traverse the centre of the field to reach the eye 

 of the observer without having undergone any deviation, and it 

 reduces by half the angles of incidence of the rays which limit the 

 field. Under these conditions the choice of a more suitable section 

 and the application of a better cementing substance would suffice to 

 give to the polarizing prism all the qualities desirable. 



The lower the index of refraction of the cementing medium, the 

 greater the limiting angle under which the ordinary ray is totally 

 reflected, and the more the dimensions of the prism may be reduced. 

 But if its index has a value less than the minimum of the extra- 

 ordinary index, notwithstanding the best selection of the plane of 

 section, it is this ray which in its turn will undergo in a part of the 

 field total reflection, and which will be stopped. Hence, as in a prism 

 of ordinary construction, a diminution in the angular extent of the 

 field of vision. The most suitable cementing substance should be one 

 whose index has the same value as the extraordinary index in the 

 section perpendicular to the axis. Linseed oil, a substance sufficiently 

 drying for this purpose, has an index (1-485) identical with that of the 

 spar ; it allows therefore of a length less than that which is necessary 

 when Canada balsam is used, and gives at the same time the large 

 field of 35°. Poppy oil, which has a lower index, allows of a still 

 greater diminution in the length of a prism, but it at the same time 

 reduces the field to 28°. 



We are now able to consider the following questions : — 



1st. What is the direction which should be given to the plane of 

 section to obtain the most advantageous size of field ? 



2nd. What inclination should be given to the faces of incidence 

 and emergence relatively to this plane in order to insure the condition 

 of normal incidence on these faces to the rays which correspond to 

 the centre of the field ? 



To reply to these questions it is desirable to consider the path of 

 the rays in the interior of the prism. Divide a parallelepiped of spar 

 in two parts by the plane A B, fig. 80, perpendicularly to the prin- 

 cipal axis of the crystal. The lines oblique to A B represent the 

 limiting angles of the ordinary ray for the following substances by 

 means of which the two halves may be cemented together. 



