THE GREAT MELBOURNE TELESCOPE. 
157 
The quantity ground otf in twelve hours 
Total number of hours at this surface 
Total quantities ground off . O401 inch. 
The total number of hours grinding was — 
Rough grinding face - - 250 
Ditto back and edge 400 
650 
Pine grinding 200 
Removing a scratch ...120 
Altering focus 9 inches 200 
520 
Total rough and fine 1170 
This is equivalent to 2050000 strokes of the machine at thirty-three per minute for 
rough, and twenty-four for fine grinding. ] 
No. III. 
Approximate weight of the principal moving parts of the Telescope. 
lbs. 
[Speculum and box 
Tube boiler-plate . 
Lattice . . . . 
Polar axis . . . 
Declination-axis . 
Cradle .... 
Counterpoises . 
Smaller portions . 
Total . . 
3500 
1300 
.1370 
3200 
1500 
1100 
4700 
1500 
18170 
This is -very near the original estimate.] 
Appendix to the Description of the Great Melbourne Telescope. No. IV. 
By T. R. Robinson, D.D . , F.B.S. &c . 
Received February 10, 1869. 
Since this Paper was read I have made several observations of the quantity of light 
transmitted by object-glasses, and determined the index of absorption in various speci- 
mens of glass. The results of some of these are in accordance with the opinion which I 
expressed there ; but others present a difference which is very satisfactory as indicating 
a surprising progress in the manufacture of optical glass. 
The photometer which I used is that of Zollnee, respecting the arrangement of 
which a few words may be useful. On looking into it, the observer sees a luminous 
