The Art of projection and complete magic lantern ... - Yesterday Image

ii6The No. I and 2 condensers are used with the object glasses of1 1 inch, „ inch magnifying power, upon objects say of | inch surface.No. 3 with ,", inch and ,„ inch object glasses and objects of ^ to ^^-inchsurface.No. 4 with jiinch object glass and objects of |- inch or less surface.No. 5 the extra front placed upon the No. 4 must be used with allhigh power objectives, this now leads us to consider theOBJECTGLASSES.The goodness of any object glasses depends mainly upon their freedomfrom chromatic and spherical aberration and upon the magnitude oftheir angle of aperture. The freedom from the former (chromatic orcolour correction) renders them good in defining power, which meansexhibiting clearly and crisply all the markings of the object underobservation.The object glasses, which we strongly advise are specially correctedfor use in projecting work, as by using a slight excess of the red raysof the spectrum, the lines of the object are shown upon the screenperfectly black and the edges of the object are not fringed with colour.This is the exact opposite to the corrections used for objectives suppliedfor micro-photography, when the blue or actinic ray has to be utilized.This will perhaps better explain why object glasses specially adaptedfor this work should be used.The second or angle of aperture which if large gives power ofpenetration, or of rendering markings upon the surface of the objectvisible or distinct.Of course, if you have a tablemicroscope with a battery of objectives,these may be used, but it will be found that many of the objectives thatperform well upon the ordinary microscope body will not yield goodresults upon the screen, owing (as above explained) to the differentcorrections, also to the mounting, or to explain more clearly, the distancebetween the posterior combination of the lenses and the screw fittinginto body of microscope or object glass holder of projecting microscope.The microscope body being about one inch in diameter only, forms as itwere a stop cutting off the marginal rays, and leaving only the centre oflens (always the best definition), while upon the lantern the image isprojected direct upon the screen, and it is disappointing, to say the leastof it, to find the centre of object beautifully sharp and distinct while theedges are almost invisible through their blurredstate.We advise the operator to obtain the best results from low power