The Microscope. 215 



iBQulje gave as focal length of the whole microscope .623, while 

 the lower principal plane was found to be 1.18 below the front 

 vertex (at M^ in the figure, that distance below V^), while the 

 upper principal plane was found at 6.794 above the eyepiece. 

 The anterior focal length is positive, being measured upwards 

 from Mj, and its focus is represented at F. The focal points are 

 thus nearer than the principal planes ; this indicates that it is a 

 virtual focus, and that tbq image seen in the compound micro- 

 scope is a virtual image. The object when viewed through the 

 microscope should be somewhat nearer to the vertex T^ than is 

 the principal focus F; it is to be at the conjugate point, depend- 

 ing on the distance of the other focus above the eyepiece. By the 

 term "one-inch objective" is signified, not the actual distance of an 

 object in focus from any part of the combination, but the prin- 

 cipal focal distance when measured from the proper principal 

 plane of an equivalent imaginary lens. In the example before 

 us the whole microscope is found to be the equivalent of a single 

 imaginary lens of .628 focal length which, according the con- 

 ventional distance of natural vision, 10 inches or 263 millime- 

 tres, will represent a magnifying power of 406. 



For both telescope and microscope, the ultimate principal 

 planes are found to be external to the instrument, one in the air 

 in front of the object-glass, and the other somewhere in the eye- 

 ball of the observer ; this can do injury, however, as these planes 

 have no actual existence, but are mere devices to facilitate our 

 calculations. 



editor;s. 



DEPARTMENT 



^>^ 



THERE is already a vast variety of microscope stands in the 

 market, but another is needed. The ingenious optician that 

 shall first supply it will have a merited reward. A travelling 

 stand is needed ; one that the owner can travel with, and one 

 that can travel with the owner. 



• In this department, some suggestions have been made as to 

 the advisability of taking the microscope on the summer vaca- 



