60 



valae of the right hand screw was exactly 50 threads to the inch, and its head 

 was divided into 10 parts ; the left hand screw was only used for adaptation. In 

 using this apparatus it must first be adjusted so that the points were seen in 

 exact apposition; they must next be separated a known distance, say -['g iiiclij 

 then place a micrometer on the stage and ascertain exactly how many of ita 

 divisions are embraced by the points. Suppose, for instance, that the points 

 were separated t'u inch, and that they were found to embrace 3^ inch of the 

 micrometer scale, the proportion which these bore to each other would be 

 obviously 3^5 in. to -,\}, le., i§^ in., or about 5| to 1, and this, multiplied by 10 

 — the length of the tube in inches — gave him, in the case of the objective to 

 which he alluded, a number which agreed within one of the power assigned to 

 it by the maker, and which he conceived to be exact. In tlie case of a f-inch 

 objective, the points set as before mentioned, included 3^0 in., and the propor- 

 tion of this to the 305 in., multiplied by 10, gives 100 as the power of the objec- 

 tive. The |-in. objective gave 350 between the points at the same distance, 

 and the proportionate number in this case was (nearly) IGJ X 10 = 165, and so 

 in the same manner the power of any other objectives might be ascertained. 

 The ease and facility with which this was done was remarkable, and it was so 

 simple that it was scarcely possible to make a mistake. The value of this method 

 also was such that the power of the eye-pieces could also be very easily ascer- 

 tained, conversely, and another advantage was that the arrangement did not in 

 any way detract from the value of. the apparatus as a calliper eye-piece for 

 measuring objects in the ordinary way. There was no necessity to be confined 

 to any special distance of the points, so that if the distance at any time were 

 found to form a fraction, it was easy to draw out the tube sufiiciently to make 

 it an even proportionate part, and add that quantity in the estimation. 



The Chairman said that all present would be glad to receive this very in- 

 teresting communication fi'om Dr. Matthews, and to examine the ingenious 

 little piece of apparatus which had been brought for their inspection ; the 

 calliper eye-piece would no doubt be remembered, and this adaptation of it 

 would add very much indeed to its usefulness. He noticed, on looking through 

 it, that there was a little difference between the two points 



Dr. Matthews explained that he had at first met with some little difficulty as 

 to the pointing, and the method now adopted was suggested by Mr. Hislop. A 

 small piece of balance watch spring was broken by a blow with a hammer upon 

 a convex surface ; the broken ends gave very sharp points, although perhaps 

 they might only appear as such when seen in profile in one particular direction. 



Mr. Ackland thought that the instrument exhibited by Dr. Matthews was 

 certainly very ingenious, but was of opinion that all it accomplished could be 

 done equally well without any apparatus at all. The first thing he would do 

 would be to ascertain the apparent size of the field of view by drawing it upon 

 paper by means of a " Beale's reflector" at a distance of 10 inches, and when 

 this was once done accurately it would enable anyone at any Lime to ascertain 

 the exact magnifyi g power of any objective. Suppose, for example, that the 

 diameter of the field so ascertained measured exactly five inches by a rule. Then 

 place a micrometer upon the stage, and read off the number of hundredths of 

 an inch filling the field, then let this number = m 

 diameter of field of view =^ v 

 and magnifying power = m 



then m := -— -r 



m X V 



