28 PEACTICAL PHOTO-MICEOGRAPHY 



obtaining lenses that are satisfactory. The additional cost will 

 always be found, in the long run, to be worth incurring. 



There are certain well-recognised tests for objectives, and 

 it is often asserted as a recommendation that a lens will 

 resolve the details of a particular test-object. This is regarded 

 as a proof of its excellence, but it is by no means a sufficient 

 or satisfactory method. In any case, the test-object should 

 be one well known to the observer ; for example, in a Podura 

 scale, or in certain diatoms, there is so much variation in the 

 objects themselves that they often afford little information as 

 to the quality of the objective. 



But where an object is known, then an expert observer can 

 tell by comparing the appearances with different objectives 

 what the quality of the latter really is. For comparative 

 purposes therefore such test-objects are of undoubted value. 



For low-power objectives it is probable that no better test 

 exists than an object illuminated by oblique light, so that it 

 shows up bright on a dark ground i.e. by what is ordinarily 

 known as ' dark -ground illumination.' This condition may 

 be obtained by putting into the sub-stage condenser a stop 

 which cuts out the central rays and only allows the peripheral 

 rays to pass ; these impinge on to the object at a considerable 

 angle, so that no rays enter the objective unless they are 

 refracted or reflected by the object. Such stops for oblique 

 illumination are usually supplied with microscopes, and are 

 adapted for use with the sub-stage condenser. With powers 

 of one inch or less, this is perhaps as good a test as can be 

 used, and with it the image should appear perfectly sharp 

 and keen, with no haziness or evidence of diffused light 

 at the edges. A suitable object for this test is a section of an 

 Echinus spine, or an unstained botanical section. For tests 

 with axial illumination, the well-known object, the blow- fly's 

 tongue, will be found suitable ; the hairs on its outer edge 

 should show sharp and black, and the points of each hair should 

 be seen perfectly sharp to the extremity. There should be 

 no indication of nebulosity or duplication of the edges of 

 the image. 



With any object-glass very useful information as to its 

 denning power can be obtained by using with it a high -power eye- 

 piece say, one magnifying fifteen times. With an achromatic 



