12 



and is able to distinguish objects much more readily than at first. 

 This is a principle which can be utilized to very great advantage 

 m connection with high-power microscope work. 'In fact, the 

 writer is of the opinion that it is this contrast between the ex- 

 ternal and internal illumination which leads so many operators 

 to use artificial light, and even in some cases to prefer working 

 in the evening. Certain it is that if the surrounding light is 

 dim and the eye is allowed to adjust itself to this dimness, then 

 on looking through the microscope, details may be seen much 

 more clearly than in any other way. It is often painful to wit- 

 ness the unconscious efforts of microscopists to bring about this 

 condition as fully as possible by means of awkard attitudes and 

 facial expression. It is not at all uncommon to see the micro- 

 scope placed in a glare of light with a strong light beating on the 

 top of the object and confusingly reflected up through the micro- 

 scope, and to see the operator sitting in a cramped position, 

 bringing his head over the top of the instrument so as to shade 

 his eye as much as possible and then often drawing the eye-brow 

 down to assist in the same object. I take it that all this is sim- 

 ply an unconscious effort to give the eye the benefit of a weak 

 extraneous light. With the present apparatus, these difficulties 

 disappear. The room is darkened. All light which could pos- 

 sibly get to the operator's eye is excluded, except that which 

 comes through the microscope. There is no light coming upon 

 the top of his object to cause confusing reflections in the micro- 

 scope. The image is a.s clear as it can be made and the eye is 

 given every facility to see this image and is distracted bv no 

 others. Needless to say the apparatus is a daylight apparatus. 

 It hardly seems necessary to argue that as daylight is the 

 light which has developed the human eye it is the light to 

 which it is best adapted. Images produced by diffused day 

 light must be the best that can be produced on the retina of the 

 human eye. This seems a sufficient argument for the use of 

 daylight and a sufficient explanation of its superiority to every 

 other light. However, when all this is said, it is not possible to 

 always secure and control this natural light so as to get the best 

 results. The following contrivances are suc'h as experience has 

 shown the writer to be very useful for this purpose, especiallv 

 m sunny climates. Outside of the microscope window a univer- 

 sally adjustable white screen is placed in a sunny position pre- 

 ferably not more than ten feet away. The surface of this screen 

 may be of any white material. It can be made of wood painted 

 white, or lined with plaster of Paris, or, what to the writer seems 

 a.most equally good, a plain wooden screen covered with several 

 thicknesses of bleached cotton cloth. It is better if this screen can 



