174 



object, and it is possible a fine definition of the flames may 

 be obtained. 



But the images of circular discs of light formed by two 

 apertures, placed before a brilliant source of light, more ac- 

 curately represent double stars, and triple apertures give 

 beautiful triples, which can be made as close as possible by 

 increasing the distance of the bright discs of light from the 

 globule. This test is so severe that I have found the aber- 

 ration corrected by an adjustment of the glasses by the screw 

 collar requisite of extreme delicacy ; so that a quarter of a 

 division, fifty divisions representing one thread of the adjust- 

 ing screw, spoiled the definition^ of Powell and Lealand's 

 one-eighth objective. 



Diffraction Rings diminish to two, under very fine correc- 

 tions. — I have counted seven rings in false adjustments. In 

 the highest degree of correction one remains. There are 

 generally several observed with the finest refracting telescopes 

 surrounding double stars. Mercurial double stars and triplets 

 form exquisite objects for testing the dividing power of the 

 microscopic objectives, and they can be readily formed of any 

 degree of closeness, and the separation of the images can be 

 readily calculated. The beauty and precision of the glasses 

 may be estimated upon exactly similar principles as that of 

 telescopes, which are valued according as they will divide 

 double stars from 5" to |" or 4" apart, under powers of 50, 

 500, or 1000 diameters, 100 being allowed in general for every 

 foot of focal length, or every inch of objective diameter ; whilst 

 for microscopes the highest working power is given by dividing 

 100 by the objective focal length. 



Triplets. — Triple artificial stars shoAV, upon a minute 

 globule, diffraction rings coalescing either circular, surround- 

 ing the triplet, or broken. As the corrections are more and 

 more perfectly adjusted the triplets become less nebulous, 

 more sharply divided, and defined as round separate discs. 



A practised eye at once recognises whether the glasses are 

 over or under corrected. The achromatic imperfections are 

 seen well portrayed, yellow fog being the worst imperfection 

 of all; good definition, sharp and clear, resembling distant 

 objects vividly cut against the sky of a bright summer atmo- 

 sphere, Avhen loaded with perfectly dissolved aqueous vapour, 

 is impossible with this kind of double fault embracing both 

 chromatic and spherical error. A little colour, especially of 

 the crimson hue, is the finest of all corrections for obtaining 



' Screw 70 threads to the inch, \ division representing \ x y\j X s^j 

 = 14^0 0^ ^'^ 't^ch change in tlie separating interval of the front glasses. 



