io POPULAR LECTURES AND ADDRESSES. 



9. A small telescope attached to the framework 

 is generally used for magnifying the object and 

 the ghost, which are both seen through it simul- 

 taneously. It is removable, and it is often more 

 convenient to do without it when the most minute 

 accuracy is not required. It is not shown in the 

 drawing before you. The only other optical adjuncts, 

 besides the telescope and the magnifying lens for 

 reading the scale, are two sets of coloured glasses, 

 which may be placed in the way of rays coming 

 from the silvered mirror to be reflected at the 

 unsilvered glass, and of rays coming direct through 

 the unsilvered glass. They are not shown in the 

 drawing, but they are essential for observation of 

 the sun. In moderately clear weather, and when 

 the sun is at any considerable height above the 

 horizon, even his ghostly image, by the second 

 reflection at the unsilvered glass, is of dazzling 

 brilliance, unless abated by one or more of the 

 coloured glasses ; and when the sun is bright, but 

 not very high above the horizon, the sea itself at 

 the boundary between sea and sky under the sun 

 is often too dazzling to be looked at in its un- 



