G. GENERAL NATURAL HISTORY AND ZOOLOGY. 271 



immersion objectives of high power, avoiding diffraction 

 fringes in the images by means of a suitable condensing lens. 

 A heliostat is not necessary to obtain good results, though 

 Dr. Woodward recommends its use as a great time-saver, as 

 without it the light must be readjusted for every picture. 

 The measurements are to be made on the negative, to avoid 

 the unequal shrinkage of paper prints. The true magnifying 

 power is obtained by measuring the distance of the lines of 

 the micrometer from centre to centre in the negative, and 

 dividing by their true distance apart. Dr. Woodward states 

 that, say twenty-five to fifty negatives, containing from 50 

 to 175 corpuscles each, can be made, and the corpuscles 

 measured, in less than a quarter of the time necessary to 

 measure the same number of corpuscles in the microscope 

 by means of a glass eye-piece micrometer, and in less than a 

 tenth of the time necessary if a cobweb micrometer be used, 

 and with infinitely more accuracy. One great source of 

 error in measuring dried blood-corpuscles has been that they 

 have been measured in only one direction, whereas a large 

 proportion became more or less elliptical in drying. Dr. 

 Woodward has little faith in the claims of those who have 

 stated or insinuated that they possess the power of dis- 

 criminatinir human blood from that of other animals in the 

 dried stains which are submitted to examination in criminal 

 cases. He acknowledges, however, that the average of all 

 the measurements of human blood that he has made is rather 

 laro-er than the averaoje of all the measurements of dose's 

 blood ; but states that it is not rare to find specimens of 

 dog's blood in which the average size of the corpuscles is 

 larger than that of many samples of human blood, and, in- 

 deed, on six of the thirteen negatives of dog's blood of which 

 he furnishes tabular measurements, the average size of the 

 corpuscles proves to be larger than the smallest average for 

 human blood, and very diverse averages were obtained with 

 the human and the dog's blood from different parts of the 

 very same drop. Tlie two tables of measurements are, of 

 human blood, from twenty-two negatives taken from nine 

 drops of blood, from eight individuals ; and of dog's blood, 

 from thirteen negatives from five drops of blood from a 

 single individual ; and of guinea-pig's blood, from four nega- 

 tives, made from different parts of a single drop of blood. 



