. The book of photography; practical, theoretical and applied. Fig. 552.— Ca3!era. justment. As shown by Fig. 551, it con-sists of three simple lenses. The middleglass is held by screws which are used inthe final assembling to set it perfectly inagreement with the other two, this workbeing done by the aid of the instrumentjust described. By its means the lens isadjusted so that it produces sharp de-finition when turned in any direction, andso that the image seen in the microscoperemains stationary while the lens isrotated upon its axis. When this is at-tained, it is certain that it wil
. The book of photography; practical, theoretical and applied. Fig. 552.— Ca3!era. justment. As shown by Fig. 551, it con-sists of three simple lenses. The middleglass is held by screws which are used inthe final assembling to set it perfectly inagreement with the other two, this workbeing done by the aid of the instrumentjust described. By its means the lens isadjusted so that it produces sharp de-finition when turned in any direction, andso that the image seen in the microscoperemains stationary while the lens isrotated upon its axis. When this is at-tained, it is certain that it will give sharpdefinition at any part of its field, but itis not yet assured that the image is flatand free from distortion. Adjustment inthese respects is obtained by varying theseparation of the glasses, without disturb-ing the axial adjustment already used for the adjustments. Fig. 553.—Special Camera fob Testing Lenses. objects are magnified and examined in thefocal plane, and, by a special device in thecamera, charts are drawn to record theform of field and the astigmatic cor-rections of any lens tested. The samecamera contains a means for measiiringdistortion, a colour test is applied byanother attachment, and the prismaticsupport in the rear is an extension cap-able of accommodating the largest photo-graphic lens ever made. Method of Taking Test Photographs. The focal length of lenses is ascertainedby means of a special photographic testing LENSES: THEIR GONSTEITCTION AND USE. 387 Camera, which makes test photographs inthe form of strips, a number of which canbe taken on one pUite for the purpose of«asy comparison. Tliese tests are for de-termining the final adjustments of lenses,and for comparing the working of differenttypes. A peculiar form of camera, con-structed to enable visitors readily and ex-actly to compare the working of variouslenses, is
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectphotography, bookyear
