A Light Source for Köhler Illumination

Abstract

THE advantages of Köhler illumination have recently been discussed by Barer and Weinstein¹, and also the difficulties involved when a single auxiliary lens is used. They describe a new microscope lamp with a variable auxiliary lens-system which overcomes these difficulties. For most purposes, however, a single auxiliary lens can be used if a sufficiently large and reasonably uniform light source is available. This can usually be obtained without serious loss of light if a piece of ground glass can be placed very close to the filament of a suitable car headlamp bulb or projector bulb; but unfortunately, in most lamps, whether commercially available or home constructed, it is difficult to arrange this. A satisfactory method is to grind lightly a circular patch on the end of the bulb, using a mixture of fine well-worn ‘Carborundum’ powder and water on a glass plate. It is not suggested that this provides so versatile an illuminant as that described by Barer and Weinstein; but it does enable an ordinary socalled ‘research’ lamp to be used for Köhler illumination with medium- and high-power objectives, while if the auxiliary lens is of large diameter it will also illuminate most of the field of low-power objectives.