---- Rectory, Hereford.
In reply to your correspondent J., I would ask if he has any photographic
apparatus? if so, the answer to his question "What extra apparatus is
required to a first-rate microscope in order to obtain photographic
microscopic pictures?" would be _None_; but if not, he would require a
camera, or else a wooden conical body, with plate-holder, &c., besides the
ordinary photographic outfit. Part III. of the _Microscopical Journal_,
published by Highley & Son, Fleet Street, will give him all the information
he requires. {557}
[phi]. (p. 506.) may find a solution of his difficulties regarding the
production of stereoscopic pictures, in the following considerations. The
object of having two pictures is to present to _each eye_ an image of what
it sees in nature; but as the angle subtended by a line, of which the
pupils of the eyes form the extremities, must differ for every distance,
and for objects of varying sizes, it follows there is no _absolute_ rule
that can be laid down as the only correct one. For _distant_ views there is
in nature scarcely any stereoscopic effect; and in a photographic
stereoscopic view the effect produced is not really a representation to the
eye of the _view itself_, but of _a model of such view_; and the apparent
size of the model will vary with the angle of incidence of the two
pictures, being _smaller_ and _nearer_ as the angle increases. I believe
Professor Wheatstone recommends for landscapes 1 in 25, or about half an
inch to every foot.