1887.] Dr Edward Sang on Four-Lens Eye-Piece 155 
the above equation takes the new form 
(3rc- 1 )&i-2w(/ 1 +/ 2 ) _ £i 
and/ 3 becomes indeterminate when both the conditions 
and 
( 3 n - 1 ) 6 , - 2»(/ 1 +/ 2 ) = 0 , 
Va + Wi+Za - J i) = 0 
are satisfied at the same time. 
If, therefore, we make 
hl 3 n - 1 
and 
• • (F) 
we may use, along with these, any eye-piece C, D, provided that it 
have 
An eye-piece constructed in this way has the several advantages 
belonging to all four-lens ones. It shows the objects in their natural 
position ; it allows of a field-bar across which cobwebs and micro- 
metric scales may be placed ; a stop between the lenses B and C, 
having its aperture equal to the linage of the object-glass, cuts oft’ 
all extraneous light ; we may introduce there a sun-screen, which 
shall not be heated so intensely as that usually placed outside of the 
eye-lens E. But this particular arrangement superadds another. 
Our inverting Huygenian eye-pieces having been all constructed to the 
same ratio n , we screw on the portion B, C, D, or uprighter as it 
may be called, and are then at liberty to use any one of our 
battery, the magnifying powers being at the same time considerably 
augmented. 
4. An Effective Arrangement for Observing the Passage of 
the Sun’s Image across the Wires of a Telescope. By 
Edward Sang, LL.D. 
At night the cobwebs of the telescope are invisible for want of 
light ; we have to illuminate either the field or the wires, so as to 
make them visible bv contrast. At noon the astronomer meets 
