776 



HANDBOOK OF PHOTOGRAPHY 



tertiary spectrum remains as residual color. Objects are rendered in their natural 

 colors with apochromatic objectives. 



The correction of semiapochromatic objectives is better throughout than that 

 of the achromatic objectives. The residual color of the secondary spectrum, which 

 is always characteristic of the achromatic and semiapochromatic objectives, makes it 

 necessary to use a filter which will exclude all secondary images when photographing 

 with these objectives. 



Tests tvith the Apertometer. — The numerical aperture of an objective may be deter- 

 mined in several ways; one of the most convenient is bj^ means of the Abbe apertom- 

 eter, Figs. 5 and 6. This instrument consists of 

 a semicylindrical plate of optical glass with two 

 scales engraved on its upper surface. A revolv- 

 ing arm, bearing a target with cross lines, revolves 

 in close contact with the cylindrical wall of the 

 plate. Near the middle of the straightedge of 

 the plate is a circular disk of silver which has a 

 small slit. An auxiliary objective is screwed into 

 the thread at the lower end of the microscope 

 drawtube. 



In operation, the plate is placed upon the 

 stage of the microscope, and the objective, the 

 aperture of which is to be measured, is focused on 

 the small slit in the circular disk of silver, using 

 anj^ convenient eyepiece. Illumination of the 

 slit is secured by directing diffused light toward 

 the cylindrical wall of the plate. After the 

 objective has been focused, the drawtube of the 

 microscope is carefully removed and the auxil- 

 iary objective screwed into position. The draw- 

 tube is then replaced and a Huyghenian eyepiece 

 is inserted. The drawtube now becomes an 

 auxiliary microscope for viewing the back lens 

 of the objective. The cross lines of the target 

 are sharply focused by sliding the drawtube in 

 or out within its collar. Measurements are 

 made by displacing the target along the periph- 

 ^~ ery of the plate until the cross lines just touch 

 the boundary of the circle of light. 

 Figure 7 A is a diagram illustrating what one sees through the microscope when 

 focused on the silver slit of the apertometer. Figure 7B illustrates how the back lens 

 of the objective appears when looking through the auxiliary microscope. The target 

 should be displaced both to the right and to the left, and the mean of the readings 

 of the outer peripheral scale gives the value of the numerical aperture of the objective 

 under test. The mean of the readings of the inner scale gives the values of the aper- 

 tures in terms of the air angles. 



Testing Objectives with the Abbe Test Plate. — For the purpose of testing the correc- 

 tion of microscope objectives for spherical and chromatic aberrations, we may use a 

 prepared test object of maximum contrast. It takes the form of a crude ruling or 

 grating and is known as the Abbe test plate. The test plate is a means for testing and 

 studying some characteristics of objectives. 



It consists of several sets of parallel lines ruled across a thin film of silver deposited 

 upon a cover glass. The cover glass is a long narrow strip of glass worked optically 



Fig. 5. — Abbe apertometer 

 bled for use. 



