206 
refracting telescope by Dollond. The breadth of the object- 
glass was 3% inches; the focal length about 4 feet; the eye- 
piece was a negative one; the magnifying power 40. 
There was a diaphragm in the eye-piece of the shape of a 
long parallelogram between the field and the eye-lenses. Out- 
side the eye-lens was a double refracting prism of Iceland Spar, 
which caused such a separation of the images that when it was 
a maximum the short sides of the parallelogram were im the 
same straight line and the adjacent long sides just overlapped. 
This was the position in which it was used. The cap contaiming 
the diaphragm was furnished with a touch-mark consisting of a 
projecting spoke. When in adjustment as above described this 
mark was parallel to the short side of the parallelogram com- 
posing the diaphragm, and marked the plane of polarization of 
the image remote from it when the whole apparatus was turned 
round so that the difference of intensity was greatest. 
A cardboard tube lined with black velvet was fixed at the 
object-end of the telescope, projecting 14 inches from the object- 
glass to prevent reflexion from the interior of the tube. 
Experiments previous to leaving England: None. 
The instrament was kindly lent by the Master and Fellows 
of St John’s College, to whom my thanks and those of the 
Expedition are due. It was borrowed by the advice of Pro- 
fessors Stokes and Adams, the former of whom inspected it for 
the purpose, the latter recommended it from his previous ac- 
quaintance with it. It was only decided to ask permission to 
borrow it just in time to have cases made to convey it and its 
stand. It was therefore obliged to be immediately dismounted 
and could not be used. The eye-piece was arranged by Mr 
Ladd in accordance with the instructions of the Organising 
Committee, and was put into my hands in the railway carriage 
proceeding from Waterloo Station to Portsmouth on my way 
to joi the Urgent. Consequently there were no experiments 
before leaving England with the instruments used. 
