WITH REFERENCE TO MICROSCOPY. 19 



and greatly influences the observer's judgment. Tliese causes may 

 also affect the length of focus, and with it the cover corrections of 

 high power objectives. Focal equation causes many differences of 

 opinion. Great difference often exists between the focus of the 

 right and left eye. In such a case, even if each has the power of 

 throwing a clear image upon the retina, those images are of dif- 

 ferent sizes, and cause confusion when combined in the binocular 

 microscope. An eye of short focus projects a smaller image upon 

 the retina, and generally has a larger angle of aperture, and 

 greater capability of resolving surface markings, but it has less 

 depth of focus than a long-focussed eye. In using high powers 

 eyes of different focal lengths often require different adjustment of 

 the objective ; this is peculiarly apparent with Dr. Piggott's 

 " Searcher," where the range of adjustment is great. The diffi- 

 culty of realising binocular vision from the above causes, as well 

 as the want of accommodation and non-union of focus and other 

 differences of the camera effect of vision, frequently cause dissatis- 

 faction with binocular arrangements ; — the matter is worthy of 

 careful investigation. Form. — The general tendency of the eye is 

 to show ultimate particles circular, and lenses rather exaggerate 

 this than otherwise. In the use of double cylindrical lenses, how- 

 ever, for examining photographs, &c, a fictitious squareness is 

 produced in two directions, which has the effect of simulating 

 greater sharpness of definition. Triangular apertures have often 

 been used in star-gazing, and diffraction rings are thereby got rid 

 of, except in certain directions. A hexagonal aperture has also 

 been used to counteract certain atmospheric effects. The existence 

 of astigmatism produces great differences in estimation of form — 

 particularly in the examination of closely-ruled lines or rows of 

 (Jots. Though long known as a scientific fact, it is only of late 

 years that public attention has been prominently drawn to it, or its 

 effects taken into consideration. It forms part of the subject of 

 Professor Liebreich's lecture on " Turner and Mulready " above 

 cited, in which he attributes to this defect the extravagances of 

 Turner's later paintings. Astigmatism is caused by the distortion 

 of the lenses of the eye from a globular to a slightly ellipsoidal 

 figure. Its effect is to elongate points into lines, squares into 

 oblongs, circles into ellipses, &c, thus giving to rows of dots 

 greater distinctness when viewed in a certain direction. Similarly 

 lines appear black and distinct in one direction— grey or confused 



