Fa 
As 
SPECULA BY HAND, AND EXPERIMENTS WITH FLAT SURFACES. 87 
effect from almost infinitesimal irregularities increases in a still 
higher degree, and it seems to me that even supposing that a 
would be sufficient to destroy its good detinition under a high 
magnifying power, irrespective of the disturbing effect of the 
atmosphere. 
By decreasing the focal length the rays cross at a less acute 
angle, and small variations in the reflecting surface have not so 
detrimental an effect. 
The correction required increases, but that does not seem to be 
of much consequence, so long as the proportion is not less than 
6 to 1 in glasses from 10 to 20 inches diameter, as it only requires 
longer work in the same direction. 
uch are a few of my observations during my working of these 
cula, and considering their weight and the care necessary to 
avoid the slightest inequality in temperature by handling, &c., I 
almost think they are the largest size that can be produced by 
hand ; but under the same process with suitable machinery I have 
no doubt that much larger surfaces can be satisfactorily produced 
under the condition that their focal length be not allowed to 
subject by an unusually thick coating of silver applied to one of 
my mirrors. The definition seemed to be impaired, and the rays 
an 
any optical defect in the telescope. 
PM yg Rice: fii aii SRO  e N I e ee e 
_ * This has lately been doubted by one of the leading opticians in a letter to the English 
Mechanic ; and rs the last number of that paper it has been giveu by one writer as 1-200th 
of an inch, perhaps (?) a misprint, 4/6/86 
