■222 



Dr Boyston-Pigott, On a new method of [Dec. 2, 



Doubtless, with a good telescope a fine spurious diffraction 

 enlargement would have been descried in the latter case. 



As far then as visual ansfle is concerned, it ousfht not to seem 

 surprising that a spider-line diminished by miniature 140 times, 

 so as to subtend an angle of nineteen seconds, should be plainly 

 visible in the microscope, even though diminished to the millionth 

 of an inch in diameter by ^ objective of 140° angular aperture. 



One or two points are worth referring to as regards the sharp- 

 ness of the definition. This wholly depends, first upon the quality 

 of the glasses employed, and secondly u^^on the mode of illumin- 

 ation. For the latter the fairy-like lines of minute tracery are 

 completely hidden by unpropitious glare. The image of the flame 

 of a lamp or of a white sky should be formed in the plane of the 

 tuehs. And when the object-glasses are used deeper for finer 

 miniatures, the corrections of both objectives must be carefully 

 adjusted, so as to produce jet black lines in the field of view. The 

 observer will thus get accustomed to the management of the light 

 and of the glasses. The experiments detailed in the table Nos. 

 (1), (3), (7), will properly introduce the most difficult feat with 

 the Beck ^oth. 



The result of these experiments to determine the present 

 limits of microscopic vision is confirmed by several considerations, 

 notwithstanding the wide-spread dogma that the limit of micro- 

 scopic vision is half a wave length ; and that only possible with a 

 very large-apertured objective. This generally received opinion 

 first originated with Nobert, who quoted Fraunhofer, that the 



expression sin x — - would become imaginary if the interval be- 



tAveen his celebrated lines exceeded a wave length. 



Further, Professor Helmholtz has arrived at a beautiful formula 

 for diffraction : 



\ 

 2 sma 



where e is the smallest distance visible between two bright lines; 

 A, the wave-length, and a the semi-angle of the observing objective. 



