LECTURE AND CLASS-ROOM. 145 



but this is not found to be the case. The objects that require 

 least amphfication are ahvays shown on the screen brighter than 

 the more minute. 



While we are considering that part of the arrangement that 

 concerns the condensation of the Hght, there is a point which 

 requires attention. The amount of heat produced by the com- 

 bustion of the hydrogen and emitted from the incandescent Hnie 

 is so great that when collected by the lenses it would spoil any 

 delicate preparation, unless some means were adopted to absorb 

 these heating rays. This is best effected by interposing a cell 

 with parallel glass sides containing a saturated solution of common 

 alum (potassium and aluminium sulphate) ; but the use of this 

 solution has been claimed as a special point in the patent of 

 Messrs. Wright and Newton, and so not to infringe their patent 

 right we can dispense with the alum and use the water only. This 

 will intercept by far the greater part of the heating rays, and those 

 which do pass will not produce any inconvenience. 



The remaining arrangements are those of the ordinary micro- 

 scope, except that we dispense with the eye-piece, taking our 

 image to the screen direct from the object-glass. We also dis- 

 pense with the plane or concave mirror, usually placed beneath the 

 stage of the compound microscope. In displaying objects on the 

 screen, the effect is much more pleasing if the margin of the iield 

 is well defined. There are considerable ditificulties in obtaining 

 this with lenses of high powers. This is because the object to be 

 shown is seldom or never in the same plane as the margin of the 

 diaphragm, and it is this which limits the illumination on the 

 screen. Even when the object is so placed that only the cover, 

 glass interposes between the object and the stage, the thickness of 

 this is sufficient to give a disc with blurred edges. This might, 

 perhaps, be obviated by placing a stop between the object-glass 

 and the screen. 



There is another point of great importance as affecting the 

 result, and that is the suppression of all light except that which 

 passes through the object-lens. The lantern should be so con- 

 structed as not to allow any light to pass into the room, and all 

 parts where reflectors or refractors are employed should be care- 

 fully guarded, and when it is possible the walls and ceiling of the 



Vol. VI. L 



