426 Mr. Howard Gruhh [April 2, 



For concave surfaces, Foucault's test is useful. I shall not tres- 

 pass on your time to explain this in detail, as it is described very fully 

 in many works, in none better than in Dr. Draper's account of the 

 working of his own reflecting telescope. This diagram will give an 

 idea of the principle of the system, which is really the same as what 

 I have described as useful for detecting want of homogeneity in the 

 substance of the glass. 



This system is extremely useful for concave spherical surfaces, but 

 is not available for convex surfaces, and only partially available for 

 concave parabolic surfaces. 



The really crucial test is, as I said before, the performance of the 

 objective when used as a telescope ; and the appearance of the image 

 not only at the focus, but on each side of it, conveys to the practised 

 eye all the information required for the detection of the errors. 



If an objective have but one single fault, its detection is easy ; but 

 it generally happens that there are many faults superposed, so to 

 speak. There may be faults of achromatism, and faults of figure in 

 one or all the surfaces ; faults of adjustment, and perhaps want of 

 symmetry from some strain or flexure; and the skill of the artist is often 

 severely taxed to distinguish one fault from the rest and localise it 

 properly, particularly if, as is generally the case, there be also dis- 

 turbances in the atmosphere itself, which mask the faults in the 

 objective, and permit of their detection only by long and weary 

 watching for favourable moments of observation. 



It would be impossible in one or a dozen of such lectures as this 

 to enumerate all the various devices that are practised for the locali- 

 sation of errors, but a few may be mentioned, some of which have 

 never before been made public. 



For detection of faults of symmetry, it is usual to revolve one 

 lens on another, and watch the image. In this way it can generally 

 be ascertained whether it is in the flint or crown lens. 



With some kinds of glass the curves necessary for satisfying the 

 conditions of achromatism and spherical aberration are such that the 

 crown becomes an equi-convex and the flint a nearly plano-concave of 

 same radius on inside curve as either side of the crown. This form is 

 a most convenient one for the localisation of surface errors in this 

 manner. 



The lenses are first placed in juxtaposition and tested. Certain 

 faults of figure are detected. Now calling the sui-faces A B C D in 

 the order in which the rays pass through them, place them again 

 together with Canada balsam or castor-oil between the surfaces 

 B and C, forming what is called a cemented objective. If the fault 

 be in either A or D surface, no improvement is seen ; if in B or C the 

 fault will be much reduced or modified. Now reverse the crown 

 lens cementing surfaces A and C together. If same fault still shows, 

 it must be in either B or D. If it does not show, it will be in either 

 A or C. From these two experiments the fault can be localised. 



It often happens that a slight error is suspected, but its amount is 



