1870s-90s? US Navy Academy midshipmen

[Sweazy21]

I recently acquired both of these photos identified as Ed. I originally thought was going to be early 1890s graduate. Now im not sure. 
 

First one is marked F. M. Zuller, and the second is M. M. Casler. 

The USNA museum has two photo albums by Zuller for classes of 1873 and 1875. I also located two photos by Casler of graduates from 1888 and 1893. 
I don’t know how long they were both photographers and what time frames their careers overlapped. Anybody can shed more information to help pin down a time frame. I would greatly appreciate it.