1873.] 



Circular Solar Spectra. 



437 



2. Various eyepieces were inserted, and the lenses A, B separated or 

 closed more or less till the most perfect definition was attained. 



3. Now, regarding the instrument as a perfectly corrected compen- 

 sating eyepiece, it is transferred to the tube of another microscope, the 

 objective of which is again adjusted by the screw-collar for the most 

 distinct definition. 



Tested on the principles glanced at in this paper, the corrected com- 

 pensating eyepiece, free from the usual aberrations, may be confidently 

 employed to test the circular solar spectrum of a minute distant solar 

 heliostat-disk, formed in its focal plane, by the objective also of a proposed 

 telescope. 



On new Adjustments for the Object-glasses of Telescopes and compensations 

 for residuary variations. 



Applied to telescopes the compensating eyepiece is adjusted some- 

 what differently. 



The instrument having been severely tested for an uncovered object, 

 and used for this purpose as a compound microscope (its front glasses 

 A, B being considerably overcorrected when tried for ten inches), the 

 telescope is directed to the solar disk formed at as great a distance as is 

 attainable by a prism-heliostat ; the condition of the diffraction-rings 

 will then display the state of the telescopic objective. 



If several eyepieces and adjustable lenses are applied in succession, 

 under precautions (such as securing a sufficiently small and distant 

 solar disk), the aberrations of the telescope can be accurately observed*. 



A careful consideration of the compensations effected in the microscope, 



(1) by separating the objective lenses, 



(2) by separating the eyepiece lenses, 



(3) by traversing the eyepiece in search of aplanatic images, 



leads to the conclusion that, provided the front lenses of the compen- 

 sating eyepiece be separable more or less, and the distance between 

 them and the eyepiece be somewhat variable, then a new correction for 

 the telescope is given by also separating the component lenses of its 

 object-glass by minute intervals. 



To accomplish this accurately, sliding telescopic tubes, one carrying 



constructed and employed ; but their correction is much more tediously attained than 

 by separable sets of lenses. 



* The eyepiece recommended to thus test the solar disk formed by a telescope is to 

 be used precisely upon the same principles as a microscope ; the greater the aperture 

 of the telescope the greater will be the power applicable, i. e. by deeper eyepieces and 

 objectives, which must have much more widely separable lenses than usual for " cover 

 correction," as regards screw-collar adjustments, if employed in order to compensate 

 for shortness of body. The aperture employed by this eyepiece in all cases equals the 

 angular aperture of the object-glass of the telescope at the focus of the eye-lens. 



VOL. XXI. 2 M 



