22 F. H. WEXHAM OX THE OBSERVATIONS NECESSARY 



out of focus the luminous point expands into a ring. If the object 

 glass is under-corrected for colour as in a single lens, the bright 

 ring appears within the focus, the outer margin is red, and the inner 

 circle green. If the lens is over-corrected the bright ring appears 

 without the focus, with the colours in the same order as before. A 

 practical knowledge only derived from these appearances can deter- 

 mine the amount of concavity to be given to the flint, or difference 

 of convexity in the crown, for obtaining the desired correction; but 

 even in the most experienced hands it generally involves several 

 alterations to secure perfect achromatism. When this is corrected 

 as far as practicable, a pale green colour only is perceptible beyond 

 the focus. This arises from the secondary spectrum or relative 

 difference in the width of the prismatic colour spaces of the crown 

 and flint, and seems to be a variable condition, according to the 

 composition of the glass employed. 



Though correction for spherical aberration is intimately related 

 to that of colour, a single lens, when finally achromatised, being 

 also nearly free from spherical error, yet in a combination of three 

 pair, when matched so as to be achromatic, this may be so con- 

 siderable as to render the object glass useless, and is oftentimes 

 exceedingly troublesome to remedy. The error may arise from an 

 improper proportion between the relative foci of the lenses — as the 

 back being too long. I have before stated that in the form that I 

 have advocated the spherical aberration is mainly corrected by 

 giving thickness' to the front lens, and by properly adjusting the 

 distance between them. In a glass spherically under-corrected the 

 light from the globule is greatest within the focus, and when set 

 out of focus, speedily vanishes and becomes diffused ; in the case 

 of spherical over-correction the contrary appearances result. When 

 the relative distance of the lenses is rightly adjusted, the light 

 spot expands equally, and is of the same intensity for a short dis- 

 tance on either side of the focus, in which the globule should ap- 

 pear with a clear bright margin. The object-glass is now in a 

 proper condition for testing the errors of construction and work- 

 manship. 



To examine the condition of the oblique pencils, and consequent 

 flatness and distinctness throughout the field, a small globule is 

 selected and brought to the edge, using the lowest eyepiece. If 

 the bright point in the centre of the globule, when a little out of 

 focus, approaches to the inner side of the concentric light rings, it 



