THE ORTHOSCOPIC LENS. - The term " orthoscopic " is derived from the Greek words opfloc right, and (fICO7rEW I see. The meaning is that the lens gives images more free from distortion than other lenses.
The orthoscopic lens is an arrangement consisting of two achromatic compound lenses separated by an interval, as shown in the figure.
The front lens is the larger, and is the same as the front lens of the present combination for portraits ; that is to say it is composed of a double convex lens of crovin glass, cemented with Canada balsam to a concave lens of flint, the entire lens having negative focal length —that is, causing parallel rays to converge to a focus on the opposite side of the lens to the origin of light.
The posterior lens is an achromatic compound lens formed of two, not cemented together but merely touching at the edges, and having a space between them in the middle, as shewn in the figure. The inner lens is of crown glass, and double concave, the flatter side being next to the front lens. The outer lens is a meniscus of flint glass, having its convex side outwards and next to the picture. The deepest concavity of the crown lens is therefore opposite to the concav ity of the meniscus. As these lenses are not cemented together but have a cavity between them, their inner surfaces may sometimes require wiping ; they are therefore merely deposited in the cell which receives them, and are fixed in their place by a countercell which is screwed until it touches a brass ring laid in contact with the outer lens. When a stop is used it is placed between the ring and the countercell, as shewn in the figure. The diamet,er of the posterior lens is about two-thirds that of the front lens, and the distance be tween them is about half the diameter of the front lens.
The posterior compound lens has positive focal length, and would cause parallel rays to diverge from a point on the same side of the lens as the origin of light. The front lens therefore tends to brino parallel rays to a focus, the back lens to scatter them wider apart'''.
In popular lang,uage the front lens is convex and magnifies, the back lens is concave and diminishes.
We would take this opportunity of observing that in optics the signs p/u8 and minus are introduced into formuht from their property of being able to represent not merely the operations of addition and subtraction, but also contrariety of position or direction. By calling lines measured on one side of a lens positive, and on the other side nemative, and affixing the signs plus or minus to the magnitude of a line, according to its position, it is possible to make one fornula in clude a great variety of different cases. The convention adopted in optics is, to call lines measured from the lens towards the origin of light po8itive, and in the opposite direction negative. In other branches of mathematics it is found very convenient to adopt similar conven tions in which the signs plus and minus indicate opposite qualities, as well as mere addition and subtraction.
The exact particulars of the compound lenses are as follow :— Front lens—Diameter 8 centimetres Focal length 40 do. (Negative) Back lens—Diameter 5 centimetres.
Focal length 90 do. (Positive) Distance between the lenses 4 centimetres.
Focal length of entire combination . . . 63 do. (Negative) Diameter of the field 54 do.
[A centimetre is about two-fifths of an inch.] These dimensions apply to the 3-inch lens, but by dividing or mul tiplying them all by any given quantity, the corresponding dimensions of any other size of lens may be obtained.
The front lens is placed with its convex side to the view, and the back lens with its convex side to the picture.
Since the front lens of this new instrument is the same in every respect as the front lens of the portrait combination, the latter may be converted into the former by removing its posterior lens and sub stituting for it the posterior lens of the orthoscopic combination, mounted, of course, at its proper distance from the front lens.