Notes. ."<::; 



determine the power of the lens, a suhsidiary lens is used to focus a bright 

 object, the image being observed by a high-power eye-piece. 



In front of the subsidiary lens is placed a plane parallel trough, contain- 

 ing a transparent liquid, such as clove oil or immersion oil. 



The bright object is focused, the lens is inserted in the trough, and the 

 object re-focused. If i\ and v 2 be the focusing distances — 



'•, - ''., 



= power of the lens as used, 



and v t - v 2 , i\ and v 2 must be measured with the same proportionate accuracy 

 as is required in n 2 - ■;/„. 



The paper contains a list of readings showing that an accuracy of 0' 0005 

 was obtained, and the author claims that with specially designed apparatus 

 the error could be reduced to 0001, and that the values of the dispersion 

 could be obtained with the accuracy of spectrometer measurements. 



To avoid the errors in the refractive index of the liquid due to tempera- 

 ture variations, the trough is made in the shape of a prism, and any variation 

 in the temperature causes the image to move in the field— thus permitting of 

 a correction being made for temperature errors. For obtaining the refractive 

 index of the liquids, standard lenses are used. 



The Specification and Measurement of Optical Aberrations. 



By C. V. Drysdale, D.Sc. 



This paper is a general discussion on the aberration of optical instru- 

 ments. It is pointed out that optical image-forming instruments fall into 

 two distinct classes— objective and subjective. To the former class belong 

 instruments such as photographic lenses, projection apparatus, etc., where 

 the image is formed on a screen, and which are therefore complete in them- 

 selves. To the latter class belong instruments such as the Telescope and 

 Microscope, where the final object is to produce a perfect image on the retina 

 of the observer, and therefore these should have their aberration defined 

 with respect to the normal eye. 



* Diffraction in Optical Instruments. 

 By J. W. Gordon, F.R.M.s. 



In the geometrical representation of a beam of light there arc two con- 

 stituent elements— the rays and the wave-fronts. The rays traverse the 

 beam from end to end and extend in one dimension only. The wave-fronts 

 lie athwart the beam and are extended in the two remaining dimensions. 

 The wave-fronts may be more exactly defined, for they are monophasal sur- 

 faces A wave-front may accordingly be said to pass through all those 

 points in the rays composing any beam which lie at a given optical distance 

 from its point of origin. . ... . . 



It thus appears that the rays intersect the wave-fronts m a beam of light 

 From the nature of this intersection the type of the beam may be determined. 

 Thus the ray where it intersects the wave-front may be a normal to the 

 surface of the wave-front, or it may meet it at an oblique angle. If the 

 ray is a normal the pencil is normal of which it forms a part, and \w have 

 the normal beam of ordinary lighl which forms the subject oi investigation 

 in what is commonly called geometrical optics. But when the angle is 

 oblique the beam is a beam of diffracted light. 



The phenomena of diffracted light arc usually grouped into two eta 

 named after Fraunhofer and Fresnel respectively. The Fraunhofer bands 



