376 Shutters for a Dome for an Equatorial Telescope. 
The turret form also is objectionable for large instruments, as 
the shutters (if made and operated like those of o_o 
become very heavy for a six foot slit; and the extra materi 
required for this form is also expensive. 
he 3d or spherical dome is almost universally used in 
America ; and from recent constructions it is plain that it can be 
made cheaply of almost any size. e new dome of the Naval 
Observatory at Washington is forty-one feet in interior diameter. 
The only difficulty then in providing a suitable shelter for a large 
instrument is in the shutters. Many forms of these have been 
proposed, to all of which there seem to be objections, and as 
the structure increases in size these objections become more 
Importan 
the following way, many of the objections will be obviated. 
The slit is to extend from the highest point of the dome to the 
cal, two 
the slit, 
the shutter must be revolved one hundred and eighty a°8 slit 
round the vertical diameter of the dome as an axis. If idian is 
1s turned southward, the whole southern part of the meridiap 
e zenit 
toward the north. To view the northern meridian, eee 
the horizon and 90—a degrees of altitude will Oe sitet 
portion of the meridian near a degrees of north zenith ¢ iss 
will not be well viewed. If any object in that place 1s 
