226 ON THE THEOET OP THE illCEOSCOPE. 



entirely miss the mark. A trustworthy estimate of a well con- 

 structed and correctly performing objective — if based at all upon 

 ascertained condition of its delineating power —can only be 

 formed by analysing every visible aberration in its separate 

 elements, and by tracing out singly each one of the whole sources 

 of error in operation. 



The principle upon which the mode of proceeding to which 

 reference has been made above, may be here generally indicated. 



As test object, is used a preparation which presents only 

 sharply- outlined black and white lines alternating with each 

 other, and lying in the same plane, so that no deviation can occur 

 in the course of the rays transmitted through it. A preparation 

 of this kind, sufficiently perfect for all practical purposes, may be 

 made by ruling groups of lines (coarse and fine), with the aid of 

 a dividing machine, on the metallic film of silver or gold fixed by 

 known methods on glass, and having no greater thickness than a 

 fractional part of a mikro-milimetre (1 mikro-mm. =^3- J^ ^ inch). 

 Covering glasses of various thicknesses (accurately measured) are 

 ruled on their under surfaces with lines -3-0 ^^ t^Vo ^^ ^^^ inch, 

 and cemented on a glass slide with balsam, one beside the other. 

 A preparation of this kind serves for the highest as well as lowest 

 powers. The illumination must be such that light may be 

 reflected simultaneously from several sides upon the object, and 

 means provided for regulating at will the course of any pencil 

 entering within the angle of aperture of the objective to be tested. 



The testing process has for its aim to bring under view the 

 co-operation of every zone of the aperture, whether central or 

 peripheral, and yet, at the same time, to be able to distinguish and 

 recognise the images which each zone delivers separately. For 

 this purpore the illumination is so regulated that every zone of 

 the aperture shall be represented in the image formed at the upper 

 focal plane by tracks of the entering pencils of light, yet so that 

 for each zone a small streak only of light be let in, and that the 

 tracks be kept as widely apart from each other as possible. 

 According to the amount of angular aperture, two or three 



