172 



In interpreting results, it is necessary to proceed from the 

 known to the unknown ; and us we can only see minute 

 ohjects through a medium, more or less false to truth, the 

 selection of known actions and reactions can alone be a 

 standard of truthful representation. 



Thus the finest objectives are corrected to show a standard 

 appearance (that of the podura) as the severest test known 

 for penetration and definition, which is utterly untrue to 

 nature ; and with a false standard little progress can be 

 made. For this reason I have sought the use of known 

 objects of sufficient delicacy and refinement suitable for 

 estimating and recording the actions and reactions of aber- 

 ration, as a check and test of theory. See Fig. 16. 



The finest glass threads offer heautiful plienomena as tests 

 of the state of the corrections. 



These may be obtained of excessive tenuity, and their 

 tendency to adhere often give interspaces of the 20,000th 

 of an inch, forming, as it were, an exquisite artificial slit. 



(«) When the aberration is very improperly corrected, 

 these threads transmit a white light, the spectral rays, being 

 dispersed in the direction of the cylindrical focus of the 

 thread are, by the confused direction of the aberrating rays 

 of the glass, commingled so thoroughly as to unite them into 

 Avhite light, and create a general haziness ; but so soon as 

 the aberration is corrected they become illuminated with 

 prismatic colours according to the particular plane of the 

 focal vision. 



Generally, according to the depth to which distinct vision 

 can penetrate, a bright band of light traverses each cylinder, 

 varying in breadth and variety of colour ; the richness and 

 distinctness testing the perfection of the corrections ; whilst, 

 if the plane mirror be usedj fine longitudinal lines represent 

 the cylindrical images of distinct objects, as the bars of a 

 window. In this way the effects of eye-pieces three or four 

 times the usual depth may be appreciated. 



In the finest examples of microscopic definition hitherto 

 known, as with some of the most exquisitely formed diatoms, 

 as seen with a l-16th objective and an immersion arrange- 

 ment, their structure cannot be seen at all, unless the glasses 

 are sufficiently corrected to show prismatic colours, especially 

 the crimson rays which possess the most penetrating power, 

 as seen in the colour of the solar disc in a London atmo- 

 sphere ; but in the study of the behaviour of minute spherules 

 of glass or cylinders, which are more easily found, it is 

 apparent that accurate aplanatism, obtained by skilful com- 

 bination of achromatic glasses, should be true to nature, and 



