PROCEEDINGS 01' THE THIRD ENTOMOLOGICAL MEETING 1045 



These dots in tlie negative are obtained by placing in the camera, 

 between the lens and sensitive plate, a screen consisting of two pieces 

 of glass which are engraved with parallel lines and joined together so 

 that the lines cross at right angles. 



There are various rulings varying from 50 to 200 or more lines per 

 inch and the finer the screen the finer the illustration, provided the 

 conditions for printmg the block are perfect. 



It is however seldom desirable to use a screen finer than 150 lines, 

 especially in India, as owing to climatic and other conditions it is diffi- 

 cult to print blocks made with an extremely fine screen. From the 

 negative a j^rint is made on a sensitized piece of copper or zinc by ex- 

 posure to light. 



The light passing through the dots of the negative renders the solution 

 on the metal insoluble so that it is covered with a mass of dots the 

 reverse of the negative. 



These dots form the printing surface after the metal between thera 

 has etched away. 



The etching requires considerable skill, as to obtain the real tone 

 value it is necessary to stop out various parts during etching ; otherwise, 

 except from a very bright and brilliant picture, the result would be 

 flat and lifeless. 



It should be explained that a half-tone block must receive from the 

 printing roller an even coat of ink all over and that it is impossible to 

 ink one part lightly and other parts with a tliick layer of ink. 



The eifect is obtained by varied sizes of dots ; the very fine dots in 

 the high lights impress only a small dot on the paper, while in the darker 

 tones and shades the dots are larger and in a given area more ink is 

 impressed on the paper than when the dots are smaller. 



A reference to the diagram illustrated on Plate 177 will make this 

 clear. 



After the plate has been etched it is mounted on wood and is now 

 ready for the printer. 



We will now consider what to avoid in preparing originals for repro- 

 duction by the half-tone process. 



Wash drawings or photographs should be of the best possible. A 

 bright clear original with all the detail clear and distinct will give the 

 best result. 



With skill good results can be obtained from flat photograj^hs but, 

 if possible, they should be avoided, as they create difficulties at every 



