USE AND MANIPULATION OF MICEOSCOPE 159 



nearest approach to a sharp image that can be obtained with 

 a lens not properly corrected. 1 



The reason why the Podura scale is such a good test, is 

 that the appearances on either side of the focus when the lens 

 is not properly corrected are of different patterns ; when the 

 lens is in correction, this is changed into one and the same 

 pattern whether the lens is moved either within or without 

 the focus. 



It is unfortunate that at present good mounted Podura 

 scales are difficult to obtain, and even those that can be 

 got are expensive. An alternative is the well-known diatom 

 Pleurosigma angulatum. This object requires a good deal of 

 practice to get the best appearance under even proper con- 

 ditions, and in some respects it is perhaps somewhat mis- 

 leading, as the appearances to be obtained are so very varied. 

 Select one mounted dry and with a frustule that is in optical 

 contact with the cover-glass ; if the frustule is itself fractured 

 so that a broken edge is in the field of view it will facilitate 

 correction. Carefully focus this broken edge so that it shows 

 what is usually referred to as the postage-stamp fracture. If 

 the correction for cover-glass thickness and for tube-length is 

 as it should be, then on getting this edge exactly into focus the 

 slightest possible movement of the fine-adjustment will bring 

 into view a black-dot pattern which should be perfectly defined. 

 If either of the conditions named are departed from, this black- 

 dot pattern will only appear if the fine-adjustment is moved 

 to a considerable extent, either above or below the point at 

 which the broken edge is in focus. If it is necessary to focus 

 downwards to get the black-dot image into view, it indicates 

 that the tube-length must be increased ; or alternatively that 

 the correction-collar on the objective must be altered in the 

 direction which would be required when the thickness of the 

 cover-glass is less than that for which the objective is corrected. 

 On the contrary, if it is necessary to focus upwards, the opposite 

 conditions are indicated; and in each case the tube-length or 

 the adjustment by the correction-collar must be altered in the 

 contrary direction. Perhaps for a beginner this object is even 

 more useful than a Podura scale, for even in cases where there 

 is but a brief experience of the appearance of microscopic 



1 See Cantor Lectures on The Theory of the Microscope, by Conrad Beck. 



