76 The Microscope. 



The results found by Bergmann, recorded in the foregoing 

 quotation, correspond to a distance of 27.77 feet, which corres- 

 pond, as closely as could be expected i^erhaps with the results 

 in the table as respects No. 1. From the table the following de- 

 ductions may be made : At a distance of 10 inches from the eye 

 a black square on a white back groimd in order to be visible 

 must have a dimension of at least y^o of an inch square, that is, 

 the angle subtended by a square ^V of an inch on a side at a dis- 

 tance of 26 feet and 4 inches, is the same as that subtended by 

 a square rko of an inch on each side at a distance of 10 inches. 

 With a tube-length of 10 inches and an amplification of 500 

 diameters the smallest black square on a white back-ground that 

 can be seen must be at least -sv-sjjou of an inch square ; and under 

 the same conditions the smallest square that can be defined 

 must be at least ^tW inch on a side. 



With the same tube-length, and with an amplification of 1000 

 diameters, the smallest similar object visible must be at least 

 tWotjc) of an inch square. With unaided vision the same object 

 in order to be defined at a distance of 10 inches from the eye 

 must be at least tts^ of an inch square. With a tube-length of 

 10 inches and an amplification of 1000 diameters, the smallest 

 similar object in order that it can be defined must be at least 

 rjisxym of an inch square. 



These results are the mean of all the above recorded obser- 

 vations. The results arrived at by the writer vary somewhat 

 from the above mean. At 10 inches the smallest black square 

 on a white back-ground that is visible to the writer must be at 

 least ^0 of an inch square ; and the smallest that can be defined 

 with an amplification of 500 diameters the smallest similar ob- 

 ject visible would be t^^swit inch square, and the smallest defined 

 Y7^o?j inch. With an amplification of 100 diameters the small- 

 est square visible would be sttjWo inch on each side, and the 

 smallest defined t^^j^joo inch on a side. 



With an amplification of 1500 diameters and the same tube- 

 length, the smallest object that is visible to the writer would be 

 at least T^osmnr of an inch square, and the smallest that can be 

 defined under the same conditions must be at least ijWjrny of an 

 inch square. It will be observed that with a whole square on a 

 black back-ground contrary to what might be supposed the links 

 of visibility and of definition are not so small. 



