PRACTICAL PHOTO-MICKOGKAPHY. 7Y 



finds a stop at the back of any objective, the aperture being 

 small enough to constrict to any notable extent the cone of 

 rays proceeding from the objective, he is advised to remove 

 that cell or stop, blackening with some dead-black pigment the 

 interior of the objective where it will probably be found bright. 

 The writer owns a ^-inch o.g. which, when purchased, had a 

 ridiculously small stop at the back. The lens was condemned 

 as very poor by several experts who examined it ; but, on re- 

 moval of the stop by way of experiment, it was found to be 

 a very good lens indeed, and has been much used by the writer 

 for a certain class of work. 



Reflections inside the apparatus. The reader is earnestly 

 advised to made sure that there are no " shiny " points or sur- 

 faces inside any part of his apparatus for photo-micrography. 

 The inside of a microscope tube is almost invariably worn 

 bright at some places, and even the black varnish inside an 

 ordinary camera is often quite capable of causing most trouble- 

 some "flares." The micro-tube should be lined with black 

 velvet, and not only should the interior of the camera be 

 dead black all over, but blackened cards should be put inside, 

 having apertures graduated from front to rear of the appa- 

 ratus, so that no light can reach the plate except that which 

 passes through the objective and forms the image on the plate. 

 This is a most important caution and is especially to be ob- 

 served when the image is projected by the objective alone 

 without ocular, for as already noted the ocular has the merit 

 of reducing danger from untoward reflections. Whether the 

 ocular is in use or not there should always be, as close as con- 

 venient to the front of the sensitive plate, an opaque plaque 

 'having a disc of aperture a shade larger than the area of plate 

 being used. T Yhv plaques may be of cardboard, wood, metal, 

 or any other suitable material, but they must present a dead- 

 black surface. 



Sometimes the edges of the apertures in the condenser-stop 

 become bright from friction, and this is specially the case with 

 badly designed " Iris " diaphragms. The writer has had con- 

 siderable trouble with this defect in one of his apparatus, and 

 warns the reader against it. Internal reflections cause uneven 



