periscopic Camera Obscura and Microscope . 373 
shorter radius ; and hence are made to converge to a point so 
distant that ' the image ( at /) falls very nearly in the same 
plane with that of an object centrally placed. 
In the use of spectacles by long-sighted persons, the course 
of the rays in the opposite direction is so precisely similar, 
that the same figure might serve to illustrate the advantages 
of the periscopic construction. For the purpose of seeing the 
extended page of a book (as at AB) with least fatigue to the 
eye, that form of lens will be most beneficial, which renders 
the rays received from each part of its surface parallel ; and 
this is effected by the exact counterpart to the preceding ar- 
rangement; for in this case the opening E represents the 
place Of the eye receiving parallel rays from the lens in each 
direction, instead of transmitting them from a distance to- 
wards it. 
There is, however, this difference between the two cases, 
that in the camera obscura a much larger portion of the lens 
is required to conspire in giving a distinct image of any one 
object ; so that the conformation best adapted for lateral ob- 
jects, would not be consistent with distinctness at the centre ; 
and hence arises a limit to the application of the principle. On 
the common construction, the whole lens is so formed as to 
give brilliancy and distinctness at the centre alone, without 
regard to lateral objects. In adopting such a deviation from 
the customary form, as I propose, in favour of a more ex- 
tended view, some diminution of the aperture is required in 
order to preserve the desired distinctness at the centre. In 
my endeavours to ascertain the most eligible form of menis- 
cus for this purpose, I have assumed sixty degrees to be the 
field of view required. But when so large a field is not wanted, 
MDCCCXII. g C 
