THE MICROSCOPE ITS MISINTERPRETATIONS. 179 



I experienced in witnessing the skill of a professional microscopist of 

 this country. In his hands, all difficulties appeared to vanish, and he 

 showed me one of the most difficult objects known, with marvelous 

 promptitude. 



But, to return to my subject : To enable the student to familiar- 

 ize himself with the true power of the microscope, and to train his 

 eyes to detect errors of vision, certain well-known test-objects are in 

 general use ; which are also convenient to test the quality and power 

 of objectives. A favorite object of this class is the scale of the Podura, 

 a minute insect, which dwells in remote nooks of dark and damp cel- 

 lars, and similar localities. 



This scale is usually mounted dry, and, when viewed under the 

 compound microscope with suitable objectives, presents a surface 

 studded with marks similar to the well-known note of exclamation (') 



This test-object has been for years the delight of microscopists 

 possessing high powers, and a sharp definition of its peculiar markings 

 as above mentioned was accepted as its true appearance and form. 



For twenty-five years this scale was under constant examination 

 by every grade of microscopists, from the grandees of the Royal Mi- 

 croscopical Society to the humble tyro, without any new or special 

 feature being noticed, when on November 10, 1869, Dr. G. W. Royston 

 Piggott, F. R. M. S., read a paper " On High-Power Definition " before 

 the Royal Microscopical Society, and surprised the members by stating 



Fig. 3. Scale of Azure Blue, placed in Position, to show False Marking8 similar to 



Test Podura-Scales. (Piggott.) 



that all these years they had been gazing at the podura-scale, but had 

 never yet seen its true markings. Dr. Piggott's paper described very 

 fully what he had discovered as the true markings, and illustrated it 

 with drawings which represented them to be distinctly of a beaded 

 character ; in fact, as dissimilar from the old accepted idea of their 

 form as contrast could depict them. 



Every microscopist was now hunting podurae, and cellars damp 

 and dismal were ransacked for the little scale-bearers, doubtless to 

 the astonishment of numerous colonies of spiders, who must have 

 been much provoked by this invasion, and thus commenced a contro- 



